Well the ethics of killing someone else to save yourself is a difficult subject. But if that's what they were trying to convey (and I don't really think it was) they didn't do a good job. They were being chased by walkers which, while relentless, or quite slow, and even hurt, I think they could have both gotten away. That bus was like fifty feet further. My feeling from that ep is that Shane panicked and doomed Otis cruelly and unnecessarily, which is why he's experiencing something like a mental breakdown right now (or perhaps that was a symptom of it).
Shane is not going to end well at the rate he's going. I feel like the writers are trying to get back to following the comic and making Shane more of a hardass slowly sliding into insanity suggests they are preparing the audience for a justified death.
The distance of the bus kinda killed it but I think it is the wrong thing to pick up on. If I was being chased by zombies while injured and carrying medicine to save my best friend's child's life, I would most definitely do whatever was needed in that situation.
In that world it is pure survival.