Why does there need to be a consensus amongst the team? There's a lot of players with a lot of different motivations, and a lot of these guys are working together/meeting each other for the first time.
And not everyone is motivated by only stopping Thanos.
Thor wants revenge. Drax wants revenge. Hulk won't come out. Strange wants to protect his time stone. Scarlet Witch wants to save Vision. Peter wants to find Gamora. They all want to save the universe as well, but there isn't just one vision on how to do this.
They show and say this in the movie with Vision and Gamora trying to sacrifice them while Quill and Cap argue against it.
I think you are really downplaying the relationships here. Of course these people want to save the universe, but they aren't robots. They are people. And your'e expecting them to easily just KILL somebody that they love.
It's kind of hard to come to a team consensus on something that is an individual choice. The mind stone was connected to Vision's life, as was the soul stone connected to Gamora's. They have more at stake, and more input on what happens than anyone else.
Also, how much did the heroes really know? The only ones who really knew and understood what Thanos was doing was probably just Gamora, Thor, and maybe Banner. No one else knew who Thanos was, or the power of the infinity stone. What were they going to talk about? All they knew is that a big bad was coming, and that they needed to stop him. It would make no sense for them to start sacrificing lives with the little information they had.
I'm not going to defend the Starlord punch too much. I didn't like it. I thought it was stupid as well. But it wasn't a dealbreaker for me. I was definitely like "wtf Quill", but it wasn't like it was completely not understandable, or even not like Quill.
When you just find out that the guy in front of you killed your girl---yea...sense be damned, you can do crazy shit. Family members go after murderers in court rooms in front of a room full of police in real life. And Quill did a similar thing in GoTG2 when he found out that Ego had killed his mother.