I recently read an article where Chris Pratt had to defend Star-lord's actions toward the end of Infinity War, where he lost it on Thanos over Gamora's death and screwed up his own plan and leading to Thanos's win. Apparently, fans were going as far as to blame the actor for what his character did, and that is insane. Pratt is just an actor, he has literally no control over what his character does.
However, this debate over whose fault it was that Thanos was victorious is still going on, and I don't think there will ever be a real answer to it. People can blame Star-lord, or Thor for not chopping his head off, or Dr. Strange, or Iron Man, or the other Avengers forever and ever. It doesn't matter, Thanos still won. Heck, I'll even go as far as to just blame Thanos. He is the bad guy, after all. He was the one who started this whole thing in the first place. Also, he was simply too powerful in the end for everyone to handle (look what happened when Wanda killed Vision; he just brought him back to take the stone). This is usually the case when people go up against a threat they have never dealt with before.