I've written much about Leng, mostly in connection to reviews and how they couldn't use the defense of high scores for the game being "perfect up until the ending" when Leng and the writing/design surrounding him is a horrendous joke on it's own.
That said I am going to be constructive since I've said everything I can about how much Leng sucked. If they were going to do the Thessia thing they should have had Leng already in the temple with the VI when you got there rather than showing up after you recover the VI, even if that would have involved some major changes to the way the VI works and needed to be recovered.
If you showed up a bit too late and the bad guy already had the thing and rabbited, that would have been a little more tolerable than the plot armor boss fight. Especially if he wound up using a small army to keep Shepard busy during his escape.
This is sort of what I mean about writing, they could have had the same basic events, but made things more palatable in the way they progressed.
That said, nothing really excuses Leng's character design or truely terrible writing. He just doesn't fit in with Mass Effect.
One conversation I was listening to involving Leng (in an MMO) was about how political correctness might have fit into the whole thing. I never paid much attention before, but the point was that there were no asian, male characters in the game, with only one being mentioned (Keiji, Katsumi's ex boyfriend) even if there were some ladies. The implication being that Leng was added into the game, along with the two latino characters, to fill out a wide range of minorities and genders, and address a rising point that while asian women get into the media for being hot or exotic, the men don't see to get much attention outside of eastern media. of course the counter point in the same conversation was that Leng is both apparently Chinese, psychotically evil, and dresses like a villain from a C-grade Japanese production, which happens to reinforce a lot of negative stereotypes, political or otherwise making it kind of counter productive if that was the intent.
I am NOT saying that any of that is true, just that it did get me thinking, because honestly for all my ranting about political correctness at times, it didn't occur to me. Truthfully with some of the things I heard about the ending being created at the last minute and being jotted down on a cocktail Napkin, I could almost see someone at Bioware being told from on high "we have decided you need to insert a male asian character into the game that gets a decent amount of screen time, and he needs to look distinctively asian make him Chinese since we already had a Japanese character" and then figuring "well, we still need a henchman for the Illusive man... I guess we can make him Chinese... and distinctively Asian... we can make him a Ninja, yeah those are Japanese but nobody will notice, and tech-Ninjas are popular, look at all those other games.... people are going to love this!". That probably didn't happen, but it's uncanny because on some levels I could actually see it after hearing that conversation and it being a last minute shoehorned addition would explain soooo much.