Ummm...
They didn't kill Negan and his crew because they're vastly outnumbered by them, outgunned, they have one of their friends as a prisoner, and so far Negan's crew has done nothing to show that they wouldn't avenge his death or kill every single person in Alexandria "just because." His crew all stood there with their guns and watched as Negan bashed in the skulls of two people, and didn't seem to care much.
And The Governor is more evil than Negan?
Are you guys watching the same show as me?