(as) mean as a rattlesnake

Exceptionally cruel, spiteful, malicious, or otherwise ill-tempered. Her dad is usually a pretty nice, chilled out guy, but he gets mean as a rattlesnake when he's been drinking. Don't go messing with that guy! He's as mean as a rattlesnake—he'll rip your head off just for looking at him funny! You'll never make it in this business—you have to be mean as a rattlesnake to get ahead.

be (as) mean as a rattlesnake

To be exceptionally cruel, spiteful, malicious, or otherwise ill-tempered. Her dad is usually a pretty nice, chilled out guy, but he can be mean as a rattlesnake when he's been drinking. Don't go messing with that guy! He's as mean as a rattlesnake—he'll rip your head off just for looking at him funny! You'll never make it in this business—you have to be mean as a rattlesnake to get ahead.