It seems like you forgot that the Fu also lost someone important to him, which was established before we were even introduced to Lu. Plus, that was his job. He literally did not know and was doing his job under the impression that Lu killed his brother, who was his best friend/love interest? It makes sense. And the author was not trying to portray anyone as a good guy, I think the author was trying to portray that anyone can be a bad guy.
No I am not buying that he was just doing his job, we had other scenes where he was doing interrogation and no one hit anyone... was it not done because they were rich or it was just not done, your guess is as good as mine.
Also I know MC lost someone but so Lu who MC knew since childhood and knew brothers had good relationship... so while our MC acting suspicious of Lu intentions make sense - him acting the way he did make no sense, the same with plot convenience of Lu agreeing to help him where he had no reason to trust him
How I am supposed to cheer for MC when he beat and pretty much tortured the person he was supposed to interrogate? Everyone acting like there is nothing wrong is just making me sick..
Then he jump with "I wanted to protect you" and "I want to help you"... there is just no way in hell LL whose face is still swollen from beating would want to do anything with these trashes. Yup trash because after acting like this there is no way they are good guys in my book no matter how much author want to pretend otherwise