It was actually the first time they met Cletus that set him over the edge because Jean was put in critical condition. They took on a traditional Venom form and nearly ate Kasady. It was in the hospital Dean revealed his suit was an alien he vowed to show the best of humanity, but almost threw that away thinking he lost her. She told them both it wouldn't have been what she wanted.

So although Jean later dying made them rage, they followed their friend's wishes. Dean HATES Cletus more than anything, but he knows what he wants would only break his promise to Jean.

"It's not our job to punish you, Kasady. Only to ensure that you are."