As someone who has been certain at the time that I wished death on others but then reconsidered my views further down the track: yes, it's entirely possible that Scar had his moments of self-doubt and regret, but going by what we know of his character I'd say those feelings probably weren't very long-lived. Even if he wasn't exactly comfortable, he got what he wanted.
It's a dark path, but strong villainous people don't let darkness get in the way of things they want.