I'm starting to tire of getting walked over lately and have been trying to be more assertive while remaining a kind person. Previously, my girlfriend cheated on a chemistry test. I told her that bothered me and she replied "Sorry, what can I say? I guess I'm not a good person."

Today (4 months later), she texted me "Be proud. I didn't cheat today when I could have."

Naturally, I'm kind of a spiteful and sarcastic person although in the past I've suppressed that out of "niceness" or whatever. I'd love to reply "Hooray for doing what's expected!" or "Be proud. I didn't kill anyone today." Obviously, that's probably unwise.

In the past, I would have replied "Yay! :) I'm happy you're not cheating :)" but I'm kinda tired of that shit. How should I respond to let her know I'm not "proud" of something that should be a given without coming across as an asshole?