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.