There's an abusive relative in my family who thinks that they can do whatever they want since their a police officer. Can I legally call the police on a cop?

I don't see why not. Even cops are subject to the law although you might need some concrete evidence since the cop, even under arrest, might still have some pull with law enforcement.

Man who cares if he is a police officer. He is a human being jsut like you and his rights are the same as yours. So if you ever feel threaten by the bastard just do what you feel is best and see that things will turn for the best!

It's the same as throwing a case to a lawyer or a judge.
You need evidence. A strong evidence.
If you already have..then that's enough.

Or posting it to social media?
It's helpful too.

depends on the country.
if it's a place where law can be bought with money then you might as well give up since the whole police organization are in cahoots.

>>5 No, it's ot useful for the police. [1]

And if you don't write your post correctly, that's a crime : the defamation.

But can't you just make him think about his behavior rather than reporting it to the police ?

And if you really want to report it to the police, go directly to the station instead of calling them. His colleagues will have a greater flexibility than if the problem was recorded and made official. And you won't be annoyed if you do something wrong.