WHY !? Not your fault that he wasn't very good or popular at his job - if I was crap at my job for whatever reason I wouldn't get a pass, I dont see why a Thai guy should be any different and if you didn't fancy him enough to off him that's not exactly your fault either.
To feel bad about that for 20 years is to me crazy - plus it was of course probably all just bullshit anyway and if you went back there the next week you would I'm sure have found him still working away.
For all you know if it WAS true that perhaps you actually did the guy a favour, as maybe his heart wasn't in the work hence why no offs and IF so he lost his job because of that non off then maybe he went back to work elsewhere, is much more happy today because of that and is in fact thanking you for that for all you know.
Either way I'm betting that 20 years later HE still doesn't feel bad about it (or even remember it or care).