One line sums up Bill for me: "I overreacted." Like he sees gunning down a church full of innocent people and shooting his pregnant lover in the head as just an overreaction. He doesn't feel sorry over it, he just shrugs like "Eh, whatever."

Evil isn’t always mean and scary. Sometimes it tricks you into complacency before it slices your throat.

I love that he admits that killing Beatrix at the wedding made him so misreable, and during his final moments he seems content with the idea of dying by Beatrix's hands, suggesting he felt he deservered it deep down. Great character writing.

I always found it interesting how Beatrix only mourned losing her daughter and nobody else. Everyone at the church, including her own husband, was just there to help build the illusion she was a normal person and nothing more. Even Bill got her to admit that it was never going to work and she would always be a natural born killer.

“There are consequences to breaking the heart of a murdering bastard…”

I would say Bill's deadliest trait is his ability to manipulate to a point where he can turn enemies in allies. He knows exactly how to disarm people without ever raising a hand, bring people under his sway and get what he wants out of them. Hence why he is called "snake charmer". I think its part of the reason the majority of the gang went seperate ways, not even his brother Bud was particularly close to him after the massacre going to far as to lie to him about selling his sword... my theory is he grew wise to Bill's tactics and decided to distance himself, not even allowing himself to accept money from Bill who is no doubt a millionaire, choosing to live an impoverished life rather than accept any help from him.

For the longest time, I always thought how odd it was that a guy as brilliant as Tarantino would misunderstand Clark Kent on such a fundamental level. Then I realised my mistake: BILL thinks that about Superman, not Tarantino.

I liked that those who interacted with Bill felt no real loyalty to Bill (outside the vipers) and was willing to help Beatrix kill Bill. It shown a light on Bill being evil enough to not have the loyalty of his teachers

Bill’s monologue about Superman and connecting him to Beatrix was unique to say the least. He views Superman from the eyes of a killer, and like Lex Luthor, he views Clark Kent “disguise” as a nihilistic commentary on humanity in general. It seems he based that opinion on watching the 1978 film. Clark is really who he is; Superman is what he can do.

Bill is such an interesting villain. I love that when Beatrix does the Five Point Palm Exploding Heart Technique he accepts his fate. He’s impressed with her for even knowing how to do it.

Bill is a really overlooked villain when people talk about villains from Quintin Tarantino films

Bill is the embodiment of the phrase “charming devil”.  When he meets his end, I found myself in the odd position of simultaneously being sorry to see him go, but still not feeling any pity for him. I think the fact that he and B.B. were “play shooting” and the very casual way he says “I shot Mommy” are also indicators he was raising his daughter to be an assassin. (Points for honesty maybe, but I think most people would have at least phrased it differently for a four-year-old.)  I love these movies, and this may be my favorite video so far.

Update: That "Bill killed O-Ren's Parents" fan theory was later debunked. Matsumoto's 2nd in command was a completely different character named "Pretty Riki," whose death was a later-added Anime action sequence in "The Whole Bloody Affair" cut of Kill Bill.

Beatrix herself confirms she left Bill because she knew he was going to turn their daughter into an assassin...
Bill is a fascinating character, he might be one of the most evil characters Tarantino ever wrote, yet, he is so cool and charming, you cant help but like him, despite some of the absolute terrible things he does, like ordering a group of assassins to beat a pregnant woman to death. In a church. A woman who happens to be her ex. And that is one, from Hanzo's reaction when he hear his name, he more than likely did some pretty nasty stuff in the past...
Anyway, one popular fan theory is that he let Beatrix kill him by the end of the movie, he gets drunk on purpose...then again, he would've killed her if it wasnt for the five point heart exploding technique, which was something Bill didnt know she could do.
Anyway, a fantastic villain, with a memorable performance by David Caradine, who to this day, I can believe was actually Tarantino's SECOND choice for the character, after Warren Beatty turned down the role (btw, Bruce Willis was his third choice).

Bill was a compelling, vile character. The pain he caused many as well as the Bride sealed his doom.

The thing too about Bill saying "I over-reacted" is that he says it, but he still shows no remorse in his voice or mannerisms.  He's actually trivializing what he did, and he shows that he's still willing to hurt Beatrix to get what he wants, and even tries to imply that she should be thankful for what he did, like he was doing her a favor by showing her that a killer is all she'll ever be.

The funny thing, we ALWAYS see only the sweet side of Bill during the film

The only time we see a glimpse of real Bill is when he shot kiddo

Everyone else know only that part of him

Bill's speech on Superman is one of the highlights of the entire film, though it is telling that if his name was Lex Luthor, it would sound exactly like something Lex would say.  He wasn't envious of Superheros, he - like Lex - despised them, because they held a standard that even the best person could never achieve.  Lex would find it disgusting that Clark was the identity he chose, because evil men want to be Gods, while Gods want to live among mortal men.

There is a deleted scene where Bill and Beatrix are walking in a village and a man claiming that Bill kill his master, began a fight and is shown just how capable Bill is (by cutting the throat of the man after killing his companions)

Bill is almost likable which makes him more dangerous