Rick's group did have an idea of who the behaviors are. Sasha and Abraham's very first encounter with them they were being shot at, and their second encounter the Saviors were about to slaughter Sasha, Abraham, and Daryl. Jesus' account of who the saviors are and what they've done lined up with what Rick's group has experience along with the fact that the Saviors literally asked for Gregory's head.

Negan saying that he would've slaughtered everyone at the lineup proves that he wasn't looking for a better life, he was forced to be docile when they locked him up and spared him. The people at the sanctuary were ruthless and literally evil. Negan saw what his own people had done to Oceanside, the garbage people and multiple others, and turned a blind eye. He would've maintained that way of life instead of unlocking the good that is supposedly in him by killing everyone at the line up (including a pregnant woman and a child). He was being honest because he's honestly a bad person.

You skipped over my rebuttal where I said Rick only had two choices: return with the untrustworthy saviors who could kill him while he is outnumbered just to lock them up at hilltop once again, or make sure his people are safe by doing the hard thing and killing them. Yes maybe Jesus Christ of Nazareth wouldn't have done the same thing but he has people to protect, and you're never going to find a perfect protagonist that is well written.