Star-lord
Thor
Dr. Strange
Iron Man
Gamora
Cap and the others
Thanos
Everyone is to blame
No one
Clearly Thanos deserves the blame/credit for what happened, but IF we must blame the heroes than it goes to Gamora because his entire plan depended on her telling him where the Soul stone was.
Presumably all he would have needed is the mind stone. That fact that he knew she knew the soul stone's location makes it a little odd he didn't go for the mind stone first.
Also perhaps mildly interesting that those are the only two stones we didn't actually see him using.
Star Lord is the only one who actively sabotages and gives Thanos the win. Without his interference he loses hard on that weird planet.
The J-man
This video blames people other than Star Lord: https://nerdist.com/marvel-reveals-w...%20to%20Thanos