Brian was PTSDing when he hit Michael... Michael invoked the bashing. It triggered Brian. He wasn't really himself when he did it.
I think it was the single worst thing Brian ever did in this series, a sign of how deeply disturbed he was psychically by what happened to and with Justin, and so much worse than any words Michael might have said to him that the constant fandom refrain that Michael "deserved" to be hit or that what Michael did was worse than what Brian did completely freaks me out.
It's like they think all that was wrong with Brian was that his woobie left him and his best fwend was mean to him.
I consider 301/opening scene of 302 Brian to be almost in the grip of a psychotic break, he was so completely gutted. And it wasn't because Justin left him; it was PTSD related to the bashing and his childhood.
Nothing else makes sense to me. Nothing else explains the hustler thing, which was FUCKING SCARY, or the depth of Brian's delusion he was fucking Justin in the opening scene of 302 or his own complete shock at realizing how deep the delusion was -- not to mention hitting Michael like that.
And nothing else IMO does justice to Brian, a man of tremendous moral principles, great self-discipline, and huge heart. To think he would just hit Michael because he was IRRITATED or ANGERED infuriates me. He could only have done that if it went much deeper than that. And I think we're clearly shown that it did.