I'm having the urge to tell my mother how I really feel about her boyfriend.
What makes you want to do that? How do you really feel about her boyfriend?
He's a nasty asshole, he left my mom when she needed him.
You have the right to have an opinion even if that opinion is negative. If he really was toxic then it sounds like he deserves to be called out.
I'm pretty mad. I want to stop them from being together.
You should try to control your anger. If you approach the situation to aggressively it may end up poorly for you. I'd recommend communicating this in a calm manner with your mother and it may produce a good outcome.