I saw a previous trailer where they said Mercer infected Heller because he saw promise in him, which while clichéd, is a valid motivation for infection. Also, it hasnt been confirmed that this is Alex, apparently one of their plot trump cards is the question "is this Mercer?" according to the devs.
I dont know why every one is asking where Pariah is, as the available footage only shows the new main character's initial motivation, and there's no way in hell that he'd know who Pariah is.
As for the "Main character becomes villain" thing, I dont know why everyone is surprised, I mean this isnt a new technique.
http://tvtropes.org/pmwiki/pmwiki.php/Main/RogueProtagonist
I also dont get why people are so attached to him. Video game franchises switch protagonists so often that we really should have expected this.
I think that that this game turning out to be decent might be more likely than you'd think, though I do understand thats it's just as likely to bomb really. I really hope they can pull off the writing, it might even be the example of ethnic diversity in main characters that the industry needs.