right of way issue. You just don't make a left like that unless the road is clear....if you hit someone while making that left it is 100% your fault as it obviously was not safe enough to make the turn.
I also think that your diagram should have some type of time chronology. I would be much easier to understand that way. Like number the cars as to where they both were at exact moments in time. It would definitely make it easier to see that he cut you off with an unsafe left turn.