Yes it makes sense. It would motivate them powerfully. But it’s been done too many times, and it’s too easy.
The rule makes you find more interesting ways of motivating them. Are they driven by bruised pride? By romance? By obsession? Anything is more interesting than rehashing the cliché Hollywood loves.
Similarly, don’t make a villain more evil by saying “they’re a dictator and also a kiddie-fiddler orremoved”. That’s just tacking on villainy.
That would be like having a right-wing US President, and saying “oh he’s also aremoved and best friends with a child-trafficker” - lazy, unrealistic writing. Make your villains more nuanced please.
it makes sense but it tends to breed empty characters with no growth potential and no real hooks for storytelling
I suppose it can. That’s why the assassin gang/teacher(s) should fill the roles normally filled by family for that character. You can tell all kinds of stories about questioning the philosophy which supports being hired murderers, or recognizing and gaining distance from the abusive relationship which turned one into a killer, etc. I feel like a lot of these kinds of characters tell very interesting stories about figuring out who you want to be and learning that your mentors or parent figures aren’t necessarily good people.