After Sidonus' death, Titus and Leandros have a brief discussion about Titus' resistance to Warp energy. Titus says that if his resistance comes into play during his battle with Nemeroth, then "so be it". Leandros then looks away and says "so be it". You may think he's agreeing with Titus at first, but he's not; Leandros is accepting the fact that Titus is probably a heretic and should have his ass turned into grass by the Inquisition for it.
Why do Drogan's own defense turrets target him? Because he's already dead. He either set them simply to kill everything shortly before his death (the daemon possessing his corpse claims this), or they recognize that he isn't him.
When we first meet Drogan, we can hear him trying to access the power source via a voice recognition system, which keeps failing to recognize the voice as Drogan's. Drogan got his money's worth on that security system..
On the other hand, listening to the audio logs seems to disprove this theory. One of them has Drogan running into the exact same problem with the voice recognition and it takes place BEFORE Drogan gets possessed. In fact this error is actually what ends up dooming him since the computer not accepting his authorisation stopped him from being able to contain the demon that was chasing him, and which ultimately killed and possessed him.
Similarly, one of the audio logs says that Drogan is keeping his recordings locked until his death to prevent others from hearing them and interfering in his work. You, of course, can listen to them because he's already dead.
If you look at it, the Psychic Scourge device looks surprisingly similar to the eight-pointed Star of Chaos. Now, remember that it was reprogrammed to open a rift in realspace to allow the forces of Chaos to invade.
Imperial Guardsmen can perform field repairs on their equipment, like the Basilisk, because situations like in the game happen so often that they were forced to illegally understand how their machines work and how to repair them. They apparently even made a field manual for that.
Drogan's armour is generally scuffed and damaged, but both the Aquila and the Inquisitorial emblem on his chest are almost completely bisected. It could just be isolated damage, but daemons are hurt by symbols of faith and devotion, thereby hinting at his possession.
In a game that is not shy about showing blood and despite wounds to his torso that even the Space Marines think are serious... Drogan isn't bleeding. His armor, though seriously torn up, appears to be perfectly clean. He does try to handwave it with "My psychic powers are controlling the bleeding", but even when his psychic exhaustion catches up to him in Chapter 8 he doesn't leak a drop of the red stuff. That would be because his heart isn't beating.
Titus' final speech to Leandros about failing as a Space Marine because he follows the Codex Astartes too rigidly makes a lot more sense when you find that Roboute Guilliman, the Primarch who wrote the Codex, explicitly stated that he could't predict everything and that there needs to be room for personal initiative on the battlefield. Without meaning to, Titus is reinforcing Guilliman's message.
If you've played Warhammer 40k before, you knew the Orks were brutal, but they were also kind of funny. This game really drives home how unfunny the Orks are to everybody who isn't also an Ork. Audio logs from civilians recount stories of their sheer terror at watching Orks dismember children and defenceless adults and laugh as they do so. Other logs reveal stories about hospitals and shelters being forced to bar their doors at the overwhelming influx of the maimed and dispossessed, and later choosing to systematically euthanise everyone under their protection when it becomes apparent Orks have gained entry to the facilities.