Let me have some fun.
Realistically speaking, they have no reason to murder (because that's what it was) the Saviors in the compound:
1) Sure, one of their groups ambushed them. But Daryl had "stolen" their truck.
2) Even if not, they could be a renegade faction, not representing the ideals of the rest.
3) Furthermore, -technically-, they were not there to kill them, just to capture and bring them to Negan. Whom as far as they knew, could have just forgiven and released them.
4) They self-inserted on a conflict between the Saviors and Hilltop. You can argue many things, but the reality is that it wasn't their conflict.
5) For all they know, Hilltop could be the bad guys, and the ones that started the conflict with the Saviors.
Old Rick would not have approved their actions.
Not arguing Rick and the group murdered those guys but...
1.) I don't think that truck was Negan's. Those guys didn't act like it was their's. I'm under the impression that whomever took Daryl's bike is not apart of the saviors.
2.) In the real world, first impression really does matter, especially when it comes to life or death situations. For the group, this renegade group are acting as Negan's representatives, especially considering the way they speak about Negan as if he is A okay with it. "All you stuff now belong's to Negan" as if Negan sends them out for this kind of work. I don't think it's too far of a stretch.
3.) They said that usually they take at least one person out to prove how serious they are. They were ready to off Abe and Sasha.
4.) It wasn't their's but it probably would be. Plus they became involved in the well-being of Hilltop the moment they realized they needed them for food. Involvement in the conflicts of allianced groups aren't odd.
5. A good point, but taking risks is dangerous in this world. They were going off of what they were given and what they knew, and what they believe was the likelihood of inevitable conflict. They were driven a bit by fear (well earned) and by hunger, and a history of dealing with the saviors already. It wasn't a "what is right, what is wrong?" situation. It was about likelihood and probabilities and making the best possible decision based on the information and experience they have. No one said it would be the perfect answer.
Old Rick wouldn't have agreed but he probably wouldn't have made it this far. I think the decision was logical even if not ideal.