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.
The first 7 episodes of Breaking Bad are very good, but don't rush through them at once. It's a show with very little in the way of happiness or humor. It's an incredibly well told story, so much so that you feel every bit of emotion that's on screen.