I told a girl who got over depression how much I loathed her out of a petty attempt to make her fall back into it
Are you serious? Why did you want her to fall back into depression?
She cheated on me and played dirty games.
While it's painful to have someone cheat on you, trying to make your ex fall back into depression by berating them is wrong. Do not say anything to her, if you cannot find something nice to say.
I don't think she'd really become depressed again, just because I said something mean to her.
Even if you're not convinced that your words could cause he to become depressed again, you still shouldn't have said something like that to her. Plus, you don't know how she might be feeling. Your words could hurt her very badly.