Mr. Ab Snopes is hard to like. He shows up basically to take part in Granny and Ringo's mule-stealing ring, but he doesn't like to take risks. Snopes makes plenty of money off of Granny's work and her sticking her neck out; he's just not so interested in anything having to do with his own neck.
At first it seems like Snopes's worst sin is selling out the Sartoris family to the Yankees, telling them where the stolen mules are hidden. But he tops it, talking Granny into taking one last job that ends up getting her killed. Leading a sweet, if mule-thieving old lady to her doom? Sounds pretty slime-tastic to us.
When the boys find Snopes and are ready to take their revenge, he is the ultimate coward. He won't even stand up and take his punishment like an adult: "'No,' he said. 'Three of you can jump on me and knock me down again, but you got to pick me up first to do hit. I aint got no rights and justice here, but you cant keep me from protesting hit'" (5.2.48).
By not fighting back or standing up to take his punishment, Snopes just shows what a worm he really is. It'd make them look bad to kill him, so what's the harm in lying on the ground and taking a few kicks?