^ oh yes, that so easy to say comfortably sat in your chair and watching the tv, but you have to be there, and feel the fear, no fear... panic, and know that certainly you are going to die if you don't do it. It was an extreme situation and he reacted with an extreme measure.it was that, or die and let the boy die too, so i say that i can understand what he did.
Anyway i think the scriptwriters made a little mistake, because moments before he asked Otis to leave him behind and go, and later he does that? a little confusing.
And answering your questions he didn't kill him directly because the zombis are not atracted by the dead, as we saw when the guy who cut himself in the arm hid under the body of a corpse not to be found by the zombis.