Shane definitely had to go. Had he stuck around, he's have killed Rick. But he wouldn't have stopped there. . .Hershel would undoubtedly be next to die, as Shane would have viewed him as an obstacle. Once Hershel was dead, Maggie would be next. Then Glen. Lori and the baby would probably die next, once Lori put two and two together regarding all the other deaths, leaving Carl being raised by Andrea (similar to her role in the comics). Daryl would be a threat to Shane's new position as leader, so Shane would kill him, or risk being taken out, himself.
Pretty much, the group would boil down to Shane, Carl, and Andrea. Andrea or Carl would eventually be forced to kill Shane.
dude you forgot T-DOGG, how can you forget him?
and i don't even know why you spoiler tagged all of that