Kai Leng's death was an epic waste. They go through so much trouble to show us that Shep is under unbelievable stress and is starting to crack. Her comes Kai Leng - risking all life, killing a friend, possibly 2, maybe a love interest, taunting, and being an epic douche while doing it. The real Villains are not something that Shep can shoot/crush/kill, but here we have a volunteer surrogate who can be subject of physical violence, and he gives us ample reason to go completely bonkers.
Instead, one stab? That's it? Hell, I was a biotic, right until then I didn't even realize I had a tech blade.
This should've been a Mortal Kombat fatality, followed by a completely berserk Shepard smashing his corpse. Probably punctuated by EDI going "Shepard, sensors indicate he's been dead for at least 30 seconds. I think you can stop now."
Instead of a perfect scene of Shep cracking under the stress, we get one stab.