I also believe that the situation was avoidable. When Shane and Otis both went into the medical trailer, I immediately knew that it was going to be a mistake because there was no one outside to arrange the escape. That's one of the things that I think makes Shane more of a "villain," it's not just that he'll do whatever he thinks he needs to do to survive or help Carl and Lorie, it's that he's reckless. Instead of making a plan ahead of time, Shane just charged in, and when push came to shove, of course he fed Otis to the zombies.
Mark is so far off the deep end, I can only imagine that him falling off the wagon would stabilize him. After Community's reverse-Porky's gag, I can only describe what happened with the Simcoe's at the end as a reverse-Walt.