I hate my friend's significant other.
What have they done to earn your hate?
I know she was cheating on my friend with his best friend.
You should let them know so they don't find out the hard way. Break it to them easily and try to respect their feelings.
I'm going to burn them both.
It is good you are looking out for your friend. You should tell them gently so they wont be too hurt. If they need help you could suggest a therapist to talk things through.