And the manner in which he eats people may actually be more horrific than simply biting, chewing, and swallowing - he tears you apart, while you're still alive and conscious, and then draws you into him gradually, brutally torturing you as you die. And after that, he gains all of your memories, and can transform himself to impersonate you perfectly.
Which just makes it all the worse, really. Imagine, for a moment, that you live in the Prototype universe. Zombies are rampaging through the streets nearly unchecked, the military authority is more interested in destroying the enemy than in protecting you, and there's a psychopath so powerful that nobody can stop him and who doesn't actively try to prevent collateral damage or innocent casualties... and the most terrifying part of it is that he can impersonate pretty much anybody he's killed. He could be your wife/husband, whom you've known and loved beyond anything else for ten years or better, and YOU WOULD NEVER KNOW. Or, better yet, your innocent child whom you've loved and cared for since their birth suddenly transforms their arm into a gigantic sword and stabs you with it, and you're forced to watch helplessly as your son or daughter murders you and begins drawing you into his/her body, and you're so confused and in shock and suddenly, it begins to dawn on you that this isn't your sweet and loving child, but a ruthless monster that killed him/her and who claimed their identity. And by the time you realize this, you've been cleaved in two, and your spouse walks into the room and you try to shout, to tell them to run, but it's too late because you can only scream in pain and horror, and you're being absorbed into this Eldritch Abomination which, by now, looks almost as much like you as you do, and your last living thought is that your family will be next and that everything you've ever known and cherished is like an ant before an utterly ruthless and diabolical God...
Alex, and the game makes sure you know it. He can be anyone, he can beat you into a literal bloody pulp bare-handed (not to mention the other implements), and the obvious fact that he routinely eats his victims. Notable examples in cutscenes include when he finds Karen Parker and when he gets back up as Taggart (with a whole piece of his head gone from a point-blank bullet).
On the other hand, imagine being Alex himself. You're a living nightmare, a monster feared by other monsters, an apex predator...and you know this because everyone you've ever killed, you know in perfect detail right down to the moment you murdered them horrifically, and their memories are yours - they are you. You'd know every little thought they ever had to justify themselves, every moment of happiness, every moment of pain and sorrow, and their final agonised screams are still going on in your mind. And yet you're going to keep killing people because if you don't, you'll never find the truth and even more people will die and they'll never stop hunting you. At first it's all right because you're mostly a sociopath, but you absorb more pain, more humanity, more guilt...
And then you realize what you are - a goddamned virus created by the spite of a complete monster, and it turns out that the only people you thought you cared about either want to use/destroy you (Karen Parker), or end up having a momentary BSOD and scream that they never want to see you again (your "little sister", Dana). And you've killed so many people, absorbed who they are and what they were, and you are them, and imagine trying to hold onto your sanity with hundreds - no, thousands, even TENS of thousands of voices in your head at every hour of the day and night, screaming in everlasting pain because you remember when you killed them without mercy or regret, and through the eyes of the victim, and in a way, YOU are the victim of your own brutality, because you have become nearly everyone you've ever killed.
Blackwatch is an organization of soldiers that have no identity, a hair-trigger temper, are incapable of any kind of remorse, and are such ruthless fanatics that even Hitler, Stalin, and Lord ''Vader'' would be awestruck. Their creed is a chilling example of what these horrific soldiers are:
"WHEN WE HUNT WE KILL. NO ONE IS SAFE. NOTHING IS SACRED. WE ARE BLACKWATCH! WE ARE THE LAST LINE OF DEFENSE! WE WILL BURN OUR OWN TO HOLD THE REDLINE. IT IS THE LAST LINE TO EVER HOLD."
Then there's that little bit of Paranoia Fuel that is Alex Mercer's reunion with Karen Parker:
Soldier: We've been compromised: Mercer knows you're on the payroll. We're evacuating. [escorts Karen to the elevator]
Karen: You know what he can do? Oh, Goddammit. He'll kill me! [elevator stops] Oh my God. Oh my God, he's here! He's in the building!
Soldier:[transforms into Alex] I know.
Cannibal babies, as in babies that eat other babies...
Mercer: What the fuck is that? Elizabeth? What is she?
These two◊ pages◊ of pregnant infected women from the Prototype comic. It's definitelyNSFW. Also, feel free to scream.
PARIAH...just..just PARIAH. Seriously, we know very little about the "kid" other than the fact that he's the virus' full potential realized and considering what we saw the virus can do, that is a scary thought all on its own, and then there's the picture from the web of intrigue with him standing next to a mutilated corpse with pleanty of blood on his hands. And finally the oh so lovely scenario thought up by one of the people you eat for the web of intrigue of what if Alex and PARIAH meet up. In his words "[[Alex and PARIAH meeting]] would be exceedingly bad." No shit doc, no shit.
People have, in fact, said that it would be the End of the World as We Know It. This is not an 'if X character meets Y character, their shared awesomeness will cause the universe to explode' jokingly theorized in the spirit of fun, this is the actual apocalypse we're talking about here.