Preventing oppression is necessary
If a Muslim sees an oppressor trying to oppress someone and if he is in a position to prevent him it is obligatory on him to do so. The Messenger of Allah (S) says:
“Help your Muslim brother whether he is an oppressor or the oppressed one.”
People asked, “O Messenger of Allah! We shall help the oppressed ones but how is it possible to help the oppressor?” He replied,
“Hold both the hands of the oppressor so that he cannot oppress. If you do it, it is as if you have helped him and such an action is a help to your brother (in faith).”
(AnwÄrun Nomaniya)