Go watch the play again fam.
I'll try to post it, but there's an angle that's damn near floor level.
Thompson is at least a foot or more behind the three point line. Ariza anticipates the shot so he jumps trying to block it.
Thompson sees him, takes a HUGE step inside the free throw line with his right foot, dips his shoulder, and goes up INTO Ariza. As he's jumping into him that's when the knee to the head occurs.
This was TOTALLY avoidable by Thompson.
At the time he did that dumb shit, he was on FIRE. He passed on the 3 to step inside the 3 point line to take a 2. TBH it looks to me like he was trying for some get back for the incident between Ariza and Curry.
All he had to do was pump fake, watch Ariza fly by him, and take the shot. He probably would have drilled it seeing how he was playing at the time.
I hate that it happened because I would like to see both teams full strength at the finals. But if you look at that play objectively, you'll see that this was all on Thompson and could have easily been avoided.