One person? That's tricky. I'd say it would really have to depend on what would happen if he didn't. If it were Naruto or Sakura, his students, or someone he's in love with, or someone he's close to (Gai?)... I can see him doing it and then possibly killing himself. Probably killing himself. I do think he'd kill any person if there was no other choice. But in a situation like Itachi had, there are multiple other choices. And Kakashi (like most people who aren't psychopathic) would do a cost-effect analysis and find an alternative. Not a happy alternative, not one without its own price, but he would be wise enough to see past the person giving him orders and understand the big picture and his place in it.

I guess the mangaka thinks its more romantic and tragic for Itachi to go through with this Terrible Thing... sure, but that doesn't make me feel sorry for Itachi. He's not the victim here, not by any stretch. There were alternatives, he just chose not to take them.