When the mentor of ruthless assassin/lone wolf cop is murdered, the assassin/cop bands his team of skilled corrupt cops/best friends together to find the one responsible.

Also, don't use / three times like that. It's too much. Matter of fact, I wouldn't use it at all, IMO.

People are right about it being wordy. I just didn't know how to put it down the way I wanted, but this is just a rough draft anyway, so I came up with a better one.

After an assassin mentor is killed, his cop student bands his friends together to hunt down the person responsible, a corrupt ATF Agent, who is out to destroy the city.

Still may be a little words with assassin mentor and cop student, drawing unnecessary attention to the character, but I was trying to emphasize around the two main characters. It does still sounds like a typical revenge film, but I've seen plenty of movies with the same loglines that have completely different meanings behind each of them.

I like your title, Richiev, if I make it a tv series. I might have to call it that because everyone gets dirty in the cop vs cop or cop vs agent movies, so that would fit in perfectly.

A corrupt cop and his friend avenge the murder of his mentor but..... so he must ...... to ........

You have a setup. But the story isn't in your Logline.