How could he be angry at me for this?

i confided in a friend that i am really afraid of being alone in the changing rooms after martial arts practice now. cause of work i now go at night and sometimes i have to shower there. so i asked him to stick around until i am done changing so i won't be alone. the idiot decides it would be funny to scare me, and waits till i am showering and sneaked into the women's locker room, he claims he did watch me as i was dressing (which could get him killed by my husband) and when i was half dressed he sneaked up on me and grabbed me from behind scaring the crap out of me... now common sense would say since i was already skittish and i am a trained martial artist i would retaliate. i screamed and aimed to elbow him in the guts, but since i am so short and he is tall i got him in the nuts. i rly didn't mean to hit him there, but in my defense he was being an idiot. he is pissed at me now, this happened yesterday btw... but i think he is being unfair, it was his dumb idea of a prank that got him hurt. and i only hit him once! i was just trying to hit whoever it was enough to stun them so i could get away. i could have gone crazy and beat the hell out of him, i was that scared