Yeah, something like that could have worked. And why did Kai Leng hate Shepard so much, anyway? Did he see Shepard as a threat to his status as TIM's #1 enforcer? Was he upset because TIM chose Shepard over Kai Leng to lead the fight against the Collectors? Or was it just a one-sided pissing contest for him?
The general point, I guess, is that you need more than just a cool look and a badass attitude to be a good villain (or even a good character). Kai Leng is, essentially, the Darth Maul of Mass Effect.
Only without the part where he ever seems legitimately threatening.
Someone who runs away from every engagement wherein he has neither an invincible (what) gunship and/or hardcore plot armor before getting utterly annihilated? Maul wasn't a deep character by any stretch, but I at least was capable of taking him seriously as someone dangerous with a lightsaber.