I've been secretly switching my coworker's coffee to decaf

This guy sits two desks away from me and for the first year he was at our company he was genuinely unbearable in the mornings and not like "haha grumpy before coffee" unbearable, more like snapping at people, being short with everyone, once made a junior coworker cry because she asked him a question before 10am. HR stuff that never went anywhere because he's good at his job. Anyway about eight months ago we had a team offsite and I ended up talking to his wife at dinner and she mentioned completely casually that he'd switched to decaf at home because of his blood pressure and that he "didn't even notice." And something just clicked in my brain. We have one of those pod machines in the office and people bring their own pods and keep them in this little basket and his were always the same brand, dark roast, red packaging. So I found the exact decaf version of the same brand and just started swapping a few out every week. He never said anything, never noticed as far as I could tell. And I think it worked?? Like within maybe six weeks he was just... normal, still not warm exactly but he stopped snapping, stopped making people feel bad for existing before noon, actually said good morning to me two days ago which has never happened in two years. I know I had no right to do this and it's a weird violation of something I can't quite name and I'm not even sure I feel bad exactly, more just strange about the fact that it worked so well and that nobody knows and probably nobody ever will, and does this make me a bad person or did I just accidentally fix a workplace problem that HR couldn't