With Shanes partner, Rick Grimes (previously presumed dead) now back on the scene, and doing a pretty good job of leading the group, Shane was starting to feel a little out of place.
Rick was sharing leadership responsibility with Shane but at the same time, Shane had lost so much.
Yes he got his best friend back but his relationship with Lori and parental responsibility of Carl had been snatched from him.
Shane must have felt lost, confused and even hurt by this sudden turn off events.
Because of the bond that Shane developed with Carl, It must have been hard for him to have to become more hands off.
I think this is why Shane shot Otis. Shane wanted to somehow prove he cared, that he would do anything for Carl because he loved Lori so much. I think there is an element of proving himself as a leader in there too.
So, Otis and Shane are on the run at the high school and all Shane can think about is, save Carl, be the hero, win back Lori and be hailed as the most worthy to lead the group. But he's not thinking rationally. "If I shoot Otis I'll be seen as strong, a survivor and I'll get all the glory. After all, Otis nearly killed Carl and that's not ok!"
Poor sweet Otis. What a disaster, what a tragedy. The attention turns from Shane the saviour to what about Otis?
Shanes moment of madness cost him his sanity and it's all downhill from there.