It's an easy to understand question if you have some distance from the subject and are not triggered by it.
If you're a cop, and you ask someone to stop because something is up, and they just don't - if they react in any suspicious way it may cause you to want to stop them.
It's going to be extremely rare to the point of 'never happening' that you come upon someone who is both 'deaf' and temporarily 'can't see due to light' so it's not going to cross your mind.
The person admitted resisting arrest - he's deaf, not blind. When officers are physically upon you, how on earth do you not know it's them?
His lawyer said he was 'blinded by the car lights' which is understandable when the cops are behind the lights, but clearly not otherwise.
So there's a lot of room for the truth here, both parties could be innocent, both parties could be guilty.
Even more nuanced: both parties could be just slightly guilty, i.e. the cops were taking a subtly more aggressive posture than they needed to, and the civilian did something he knew he shouldn't do, which was actively resisted arrest when he for sure knew they were cops.
So cops needlessly put hands on him but still without intention to do anything, he resists which results in escalation, they fall to the ground, he breaks his legs and then you have a headline.
The story feeds into the preconceived notions that people have, people are overwhelmingly more likely to fill in the blanks with their narrow views than they would be to consider the multitudes of nuances that lead to a situation. It's a rorschach test.
That said: I would say the '4 month' bit is outrageous, we don't quite know what happened, maybe the toss-up between the police and civilian got quite serious but I can't fathom why he was not out on bail.
Edit: I will also add, that the 'firing' of the cop is also meaningless in terms of determining culpability. The cops firing will be as ultra-political as when they close ranks and defend themselves, and is further away from actual justice than anything in a court of law. If the cop brought in a bad headline for the police force, he becomes a really easy target for the leadership as a potential scapegoat and political cover. "Oh look, we've reformed!". The details of the situation may be more complicated.
It's really odd how these institutions work, they are so deeply politically entrenched and work on the basis of power, it's almost as if they are incapable of acting with thoughtful nuance. Advancement in these systems comes entirely from 'playing the game' and having a stellar record on paper and that's that.