While at the hideout, Desmond seems to insinuate that he's never had any experience as an assassin. However, in Assassin's Creed, Desmond retorts Dr. Vidic's statement that "he's an assassin", saying that he "doesn't do that, not anymore." Desmond also lived amongst assassins during his early years, it's only safe to assume that Desmond has some experience as an assassin, unless, as the story states, he ran away from the commune too soon to learn anything valuable.