One of the producers said on Talking Dead that Karl didn't want to kill his followers.
He did kill two of them but by not killing Negan, someone will pay the consequences.
Megan makes his followers kneel and be grateful for the few crumbs he tosses out to them. He gives them points for showing loyalty.
But if he's oppressive enough, I don't know how some of them don't conspire to kill him. It's not like he's some intimidating figure with his inability to fill in those skinny jeans.
While he's liquored up and screwing his harem, he could be so easily got.