I hate my best friend's girlfriend
Why do you hate your best friend's girlfriend?
I just hate her to death, I don't even know why.
It's okay not to get along with your best friend's girlfriend, as long as you don't let it affect your relationship with your friend. Talk to your friend about it so that you're on the same page.
I am going to tell him I hate his girlfriend.
You can talk to your friend but be careful not to be rude about his girlfriend. If you can be polite he will feel more calm.