I stepped on my friend's glasses because he left them on the floor, so I don't think I should have to pay for them.
But you stepped on them right, so why don't you think you should pay for them?
Because that shithead was supposed to be more careful with his glasses.
Well, it sounds like you're in the right here.  It was his mistake leaving them where he did.  People should be careful about where they put their glasses and should avoid putting them on the floor where they can be easily broken.
Really? I don't have to pay for them.
You shouldn't have to pay for them, but you should still apologize to your friend for stepping on them regardless if it was your fault or not because it's a sign of respect.