These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Based on the back story, one could infer that the ISA and UCN are the villains, for their unfair treatment of the corporations sent to colonize other planets for natural resources, especially to the Helghan Corporation. The Helghast, on the other hand, were trying to break off from the UCN's corruption but fell victim to Peace Through Superior Firepower, which forced them to flee to Helghan, adapt and become the militaristic culture that they are in order to survive and resist the UCN's tyranny. Admittedly, while becoming tyrannical themselves.
Anti-Climax Boss: One can use the flamethrower to burn Radec in 2, and his boss battle will then only consist of him screaming and flailing in agony (leaving him unable to attack) whilst the player continuously shoots him until he runs out of health, which then triggers his death cutscene.
Big Lipped Alligator Moment: In the opening cinematic of Killzone 3, Radec and Stahl shoot a man for no apparent reason. It is never touched on again.
Best Level Ever: Killzone 2 is practically made of them, highlights include:
Beating back the assault troops that have attacked the New Sun.
Taking control of a small mecha and tearing through the Badlands searching for survivors from the previous level.
And the last level where you storm Visari's palace.
The entire stage in Killzone 3 when you work to bring down the MAWLR.
The final segment of the Jungle level, where hundreds of Higs start swarming you, slowly pushing you back in a massive gunfight.
The titular Mercenaries in the PS Vita game. They work with both the ISA and the Helghast and only care about one thing: money. They aren't concerned with who's right or who's wrong in a war; the only thing they care about is who pays more.
Complete Monster: Chairman Stahl, like most C Ms, he just exudes an aura of hatred and disrespect for everything in existence. He is utterly without remorse for any and all atrocities he performs, from testing experimental weapons on P.O.W.s to laughing as he kills his rival in a particularly gruesome manner. And he is entirely convinced of his own superiority over everyone from his soldiers to the protagonists. They're all tools to be used and/or destroyed, in his mind.
Rico, who's a loudmouthed, annoying Jerkass who does a nice job breaking things (see above). In addition, his AI can be summed up as "get shot, and get shot often", and the player is NOT allowed to abandon his injured ass at any point for being stupid, he MUST be revived. He is severely disliked by the fandom as a result. This could be intentional - to offset Sev's measured approach, and ultimately show the futility of aggression for aggression's sake, as well as how blindly running in like an idiot is a poor tactic. Either way, the fans hate him and it doesn't seem that the developers are listening to complaints that Sev should kill him off already. The inclusion of a major role for him in the 3rd game is likely to be a sales deterrent.
In the third game, the plot can be almost completely summed up as "Rico is always right". While Captain Narville is trying to save his men from getting killed senselessly by ordering retreats and ceasing of gunfire, Rico instead keeps insisting that they go on the offensive. However, Rico ends up being the one who the team listens to, and Sev even states that Rico's ideas are the only ones that will guarantee a chance of survival. This is the same Rico that not only got Garza killed, but also killed Visari when the mission was to capture him alive.
Garza is pretty popular as well, mainly because of his laid-back personality and witty wisecracks.
Jammer has also become fairly popular, with people doing artwork of her and writing fanfics about her.
Kowalski, mainly because he looks like Dusty. Ironically his model was a left over from the 2007 version of Killzone 2, as visible in the second trailer, in which he was the original model for Garza, with Garza's model being used for a rookie replaced by a generic soldiers in the final version. This means that his model actualy precedes Dusty.
Lucas and Echo have become greatly memorable for their roles in Shadowfall. Both were willing to defy their sides' orders in order to do what's right and stop the Vektans and Helghasts from starting another war that would destroy them both.
Evil Is Cool: Say it with me: "Can we get a Helghast campaign this time?" Killzones 2, 3, and Shadow Fall have the Helghast in awesome-looking full-body armor, while the ISA... look like modern-day troopers with modern gear. At least the first game had some futurism for the ISA.
Game Breaker: The Helghast Rocket Launcher in the first game (especially in multiplayer). Its primary fire was bad news, but its secondary fire launched all four barrels, and had a wide area of effect.
Rico's machine gun. Not only does it do an ungodly amount of damage, it has a rocket launcher as its alt fire.
Luger's sub-machine gun. It did better damage than Jan or Hakha's rifle, and had much less recoil to boot, making headshots a snap.
Goddamned Bats: Helghast Shock Troopers in the second game. Jesus fuck, stop running around and take your lead suppository like a man!
Hilarious in Hindsight: As the events of Killzone 3 show, the ISA were in no position to safely extract Visari if they had taken him alive, and his death ended up fracturing the Helghan leadership and putting Stahl in a position where he was able to destroy the entire freakin planet!. So in a weird way, Rico shooting Visariactually worked out, although Rico had absolutely no way of knowing that at the time.
Killzone received a great deal of hype when video game websites released previews and rumours about the game, dubbing it a "Halo-killer". Upon release, the game received average reviews, with frame rate issues, bland enemies (there was a grand total of three enemy types, and four enemy model - one of which is only used in one level), jumpy controls, and glitches being cited as problems.
According to the Bungie podcast, the developers at Guerilla told them that they wanted to make Killzone a good game, but never set out with the intent of making a "Halo-killer". A Meddling Marketing Department was responsible for that moniker, which they felt was no better an idea then when Dark Cloud was hyped as a Zelda-killer (and see how well that worked.)
Killzone 3's ending. Stahl's own superweapon is used by the player's forces on Helghan, and in the aftermath, Jammer notes that the entire planet has fallen silent. How a hinted-at Killzone 4 will be made with the Helghast presumably annihilated remains to be seen.
A scene shown during the credits implies that a few Helghast managed to survive.
Shadowfall confirms that some of the Helghast managed to evade their homeworld's destruction.
Lawful Neutral: As a whole, the ISA are this. Though they aren't purely "good" by any means, they are duty-bound to carry out the UCN's laws by any means, be it by peace or by force. Also, their entire motivation for invading Helghan in Killzone 2 was to gain retribution for the casualties they'd suffered on Vekta, and to remove Visari from power and thus end an already bloody and brutal war that they had forced on them. Also, at least a good number of them take international treaties very seriously, evidenced by Narville's disgust at Stahl for killing a POW. "We are prisoners of war! We have rights! You've just violated the Stockholm Treaty!"
Colonel Radec is supposedly a dark take on Erwin Rommel. Both excelled with tactics, but are merely passable with strategies and both earned the respect of their opponents. They differ, however, in their reasons for committing suicide. Rommel committed suicide to protect his family from the Nazi Party due to allegations of his involvement in Hitler's attempted assassination, while Radec wanted to avoid being captured by the ISA. Also, Rommel was a chivalrous man under whom the Afrika Korps had no war crime to their name and he somewhat opposed Hitler, while Radec would execute his own men at the drop of a hat, possibly punt a baby into a mine field if it got his job done and he supported Visari all the way.
The repetitive Helghast dialogue from the first game.
Somewhat parodied when you first use the sniper rifle against a Helghast ambush. An infantry soldier's death (the guy on the rooftop) causes an anguished (and somewhat annoyed) "NOOOOOO!!! NOT HIM!! He was just a boy!!"
In 3, somewhat. Rico goes through Character Development so he's not as much of a jerk. Plus, he is more helpful in combat this time around, since he can now heal you whenever you get shot to the ground.
Killzone: Shadowfall introduced Lucas Kellan, who is perhaps the most human and likeable character in the ISA (next to Sev and Garza). Throughout the game, Lucas constantly worries about whether or not he's doing the right thing by fighting the Helghast.
Retcon: In the backround info of earlier Killzone games it was stated that the population of Helghan is around 23 million, a realistic number given that it was founded by only 1 million people 100 years before. However in Shadowfall's intro it's noted that around a billion died from the catastrophy.
Rooting for the Empire: The Helghast are way more popular than the ISA for various reasons. Sony realized this for Killzone 2, and made them the focus of the promotion, and much more explicitly sympathetic. Shadow Fall goes even farther to humanize them, and to point out the ISA doesn't have clean hands themselves. Also, the last mission lets us play as one for the first time.
Rico, probably the most hated character in the series amongst the fandom, due to both his attitude and his Artificial Stupidity as an in-game ally.
The ISA as a whole is this to many Helghast supporters. While people like most of the main characters (including Sev, Templar, and Garza) the ISA's most defining trait is just your average modern solder, in space.
Sinclair, he constantly ignores Kellan's warnings regarding Tyran and Stahl's plans simply because Kellan is willing to work alongside a Helghast, which causes Sinclair to get an entire ISA fleet destroyed by Stahl's superweapon. And at the end, Sinclair kills Kellan after the latter refused to go along with his plans to start another war with the Helghast.
Scrappy Mechanic: Opinion was divided on the "weighted weapons" of Killzone 2; some found the acceleration and momentum added a realistic feeling to shooting, while others found it made aiming feel clumsy. It didn't return for the sequel.
Strawman Has a Point: If you read the backstory of the Killzone universe, you'll find out that the Helghast are the way they are becuase of getting royally screwed by the ISA. Taking this into account, much of the Helghans' hatred and spite of the ISA is pretty well justified. This may well be deliberate on the part of Guerilla.
Surprisingly Improved Sequel: After months of being hyped up as a "Halo killer" (that's what it was often referred to), a good deal of Hype Backlash set in when Killzone for PS 2 turned out to be an ok-ish shooter with numerous issues (sluggish controls, low enemy variety, repeated voices, graphical issues, inconsistant framerate, etc). The game attracted a devoted fanbase, but in general became almost a byword for hubris in the FPS genre. With the sequel, it aimed to be the Killer App for the PS3, which was a slightly less lofty goal than dethroning what could be the most popular FPS series of the decade. But this time, they delivered. Critics loved it, as did players. And Killzone 3 has been received in some circles as an Even Better Sequel.
In Mercenary, Justus Harkin saw his parents getting killed by the Helghast. It doesn't help that Justus is a child.
Similarly, In Shadowfall, Lucas watches his father get killed right in front of him. Like Justus, he was also a kid when it happened.
Also in Shadow Fall, when Kellan infiltrates New Helghan in order to assassinate Tyran, we get a glimpse of what life is like on the other side of the wall, in which we see Helghast civilians living in ghetto-like conditions while being monitored constantly by security forces. After five games in which the Helghast have been portrayed primarily as faceless Mooks, seeing them as actual people for the first time, scared and just trying to survive is a truly sobering experience.
Anders Benoit in Killzone Mercenary. And rightfully so. He uses the Carapace shield, a 3-round burst sniper rifle and spends the first half of his fight on top of his Intruder. Once you grounded him? You have to stun him by shooting fuel rods Justus raises. And for the entirety of the fight, he can call for backup. On the bright side, you can scavenge their ammo.
The ISA machine gun. It now needs to reload and doesn't shoot rockets, on the plus side it no longer overheats if you fire it for too long.
The Helghast pistol also lost its burst fire mode, the Helghan assault rifle lost its Masterkey-like shotgun attachment, and the Helghast missile launcher was reduced from 3 tubes to one tube, though that's probably in the name of balance as the original ones Secondary Fire modes were a bit over powered.
The Helghast machine gun is now extremely inaccurate, quite unlike in the first game, and you can't even aim it at the risk of making it a Game Breaker. They gave it pinpoint accuracy again in 3, but only when crouched or shooting from cover.
The Helghast Assault Rifle in 2 lost it's scope and was downgraded to iron sights, while being generally average. In 3, they added the scope back.
The series went from a relatively hi-tech space shooter with four playable characters and a well-armed force fighting off a sudden incursion, to suddenly making the ISA horrendously inept and unable to fight back.
Visual Effects of Awesome: While the first game was a victim of Special Effects Failure due to its rushed development schedule and the weaker tech of the PS2, the the other two games became prime examples of how powerful the PS3 can be. Shadow Fall looks to bring this trope to another level on the PS 4.