I think that's how the curse works - every time a cursed enemy hits you, you get cursed. Every time you kill a cursed enemy you get cursed.
So if you kill him with melle you will get cursed twice (once for being hit, once for killing him).
If you kill him with first strike, you only get cursed once, because you don't get hit back.
Same with fireball - you get cursed for killing him, but you don't get hit back.