shane was just tired of bein in the "new" shadow.......rick's. after leadin the group so far (which we only had bits and pieces of), shane WAS the leader and savior of the group, made all the right decisions, had the woman, AND had a "son" that looked up to him. now rick has come back, is startin to take the power that shane has held (the group, more or less is transitionin to rick's leadership and power), and shane sees it as doin all the work for someone else to come and take applause for.
not to mention that he finds out that lori is pregnant, and she refuses to admit that it's his. she tells him it's rick's child. he's split between his loyalty to the group and rick, and his loyalty to his new child/lori (and other child, carl). i mean rick "was" dead for so long.
the world has literally fallen down around him. after lori refects him (he was in love with her since high school remember), and the group starts (in his mind) to reject him, he has no other choice but to "keep" the group that "he created" together by doin the only thing that is logical.................take out the shadow-bearer.
ETA: same as with spider tatoo in the prison............just males bein males, and pissin on stuff they think they own, or is theirs.
"they may get me in a rush, but not before i turn your head into a canoe....." -Kurt Russell (Wyatt Earp) in Tombstone
"if it was up to me, i'd like to see, this country run,
the way it used to be, the way it oughta be, just like it's done, OUT HERE.........WAY OUT HERE." -Josh Thompson, country music singer http://www.youtube.com/watch?v=D0sYnro_3Rc
PEOPLE TEND TO ACT LIKE SHEEP, BOY I LOVE MUTTON! -Me