I don't think calling a black person aggressive is racist, but I do think as there is a stereotype of black people being aggressive it can be problematic.
I mean that people might notice aggressive behaviour more in a black person because they expect it (even subconsciously) and also they might interpret a certain behaviour as aggression in a black person when they wouldn't in a non-black person.
Also I think agression in a non-black person may be more like to be excused. Or the agression may be looked at as smaller part of their personality, where it may be the main focus on a black person.
That seems to be happening here a bit I think. Serena's aggression is being overly focused on and the racism she has suffered (which was meant to be the point of the thread) isn't getting much of a mention.
I feel like the point has been made that Serena has acted in an aggressive manner in the past, and maybe she isnt a very nice person, but maybe it's time to move on from that point. Although I am curious as to what happened with that line court person she lost her temper with. Not saying there was an excuse for her to react so badly btw.