If that poster was being honest about it (a huge “if”) the most logical explanation for somebody empathising with the perpetrator of an assault but not with the victims is that they’re extremelly biased in a tribalist way and hence fully empathise with those of their tribe butnot at all with those a member of their tribe would be angry and violent against.

Or to put things in simpler terms: Nazis can really understand and share the anger that makes other Nazis be violent, never the point of view of the victims of the Nazis.

When seeing a situation like this a normal person tends to empathise with victims quite independently of who they are and not try and imagine reasons to excuse the violence of the bully.