Yes it is possible the reaction was caused by a racist remark. Yes, the player in question made a fascist gesture.
That is no excuse to assume that said racial offense actually happened, and also shouldn't be an excuse for such a violent outburst. There are better ways to handle racism, and still I see justification of violence by who claim to be on the side of "love". I also find quite funny that "never blame the victim" is deeply relativized whenever the agressor is perceived to be as part of a "minority".
If a mob lynchs a rapist, it's savagery and wrong. But if there's a slight suspicion of a racial offense then it's ok for the target to react violently? Make up your mind already if you want law to determine the punishment or justice to be made by your own hands. You can't just cherry pick that according to your arbitrary view of the world.
We live in a age where we are defined not by our actions, but by the group we are arbitrarily placed in, defined by race, gender, sexual orientation etc.
Nowadays, a man can't debate a woman without being accused of "mansplaining". So, paradoxally, men are actually coherced to treat women differently in order to not be accused of sexual prejudice.
Also, in 2018, if you are assaulted by a black men, that's because you are a racist, no questions asked.
Racism exists, sexism exists, homophoby exists plus a lot of other nasty stuff. But throwing accusations randomly and in the lack of any evidence is not going to make these problems go away. Ironically, that's a rather fascist thing to do and only adds fuel to the fire.
For those of you defending the agression over a preassumed racial offense, it's time to use your brain cells.