Recently I've played dead space and some other survival horror games like Alien Isolation, and just about every single time you see someone in there, they're somehow invincible. Like, the fuck, that fat idiot inside that VIP Area there has somehow managed to lock himself in that room for what, weeks? Months? And somehow never did any of the necromorphs even notice him? And that crazy scientist guy? He can just run around in the ship? And what about alien? The medic dude, like, there's a vent right in the room next to him. And he has a broken leg, so the alien can even smell his blood. Also, what about the marshal? Well, thats kinda fair game, he has an effective weapon after all.
The marker wanted to stay off of Aegis VII, actually. Their entire goal (as we learn in 3) is to spread the Necro infection and eventually make a Brother Moon. It's likely the marker kept attacking Isaac in an attempt to stop him from putting it back on the planet that was about to fall apart. Isaac's mental infection isn't hardcore until DS2, either, so he's not being manipulated very hard yet
Dead Space gets fucking complicated if you read the extended lore in all the games holy shit I sound insane
Mercer is the guy making the invincible Necro. In the remake he kills himself willingly, in the original he's "betrayed" by the marker. The guy helping you is June, poor bastard