I'm wondering if Bob wasn't originally a member of Terminus and he walked around to recruit people to Terminus. That's why he was alone before and said he was the only remaining survivor of two groups. That's why he drank all the time. That's why he was last in the line at the trough to have his throat cut. That's why he was begging Gareth for the lives the people there. And that's why it was "cosmic justice" that he was caught first.
These thoughts came to mind after Gareth called him, "Bob." Like, how did he know his name? There's a history there.
And, yes, I think Bob is infected. Him getting bit couldn't be a red herring. It follows the Dale story arc from the comics, I think.
I don't think the preacher is pretending to be anything he isn't... which is a scared little pussy who will do anything to save himself except overtly kill a walker or another human being.