I don't recall Shane ever killing somebody just for the sake of killing them. He had a completely legit reason to kill Otis. He only began to lose it after Rick showed up, take Rick out of the equation and we have a great leader! I feel like a lot of people look at Shane like a monster because he was made out to be an antagonist. Sure, he was hot headed. But, if you were in his group... he kept you alive. Excluding his problems with Rick, Shane didn't wish harm on his group.
Randall... Come on. He was some random guy, on a rooftop trying to kill you. So, instead of taking out a VERY major threat... we'll save him and bring him back where we sleep. No Rick Grimes. You've gotta step out of cop mode, and enter zombie mode.