I think something fundamental about Charles Vane, and why he is so formidable in a fight (and generally feared and respected) is there has never been a fight he's entered into where he hasn't been willing to kill his opponent or die if they beat him. When fights end and nobody dies, it's a happy sort of accident. It folds into honesty as one of his core character traits; he doesn't posture. He's not going to fight someone to show off or prove a point, he's going into it with the very real determination to either win or die.
Aha! You say. But he wins the fight with Albinus on Lumberjack Island and doesn't kill him.
Well, I say, look what it cost him to do that. The price of mercy for Albinus (Ranger Crew EP title) was he had to literally rise from the grave, metaphorically (or literally, if you want to lean into the subtle supernatural undertones of the show). It's the conclusion of his opening arc of not being who he is, and instead being who he thinks he's supposed to be. It's why he doesn't take Max away himself but asks Jack to do it, which leads to her recapture and further abuse (because Jack is unable to stop the crew where Charles may well have been able to), it's why he goes to the man who owned and abused him to beg his help and offer him payment rather than killing him outright and convincing the men there to follow him out of respect.
Incidentally I think this (never entering a fight without intending to kill or be killed, to win or die) is also why Anne is a similarly unstoppable force. And, though I love him, it's why Jack isn't in the same way, though he's certainly no pushover. Jack would enter a fight with no intention of it ending in death, Charles and Anne simply don't even think of that as a possibility
Edited to add: when he steps in to stop Teach from killing Flint, you're absolutely right in that he isn't trying to kill anyone then. But I believe he's still fully putting his life on the line in that instance as it also is a major point in a different developmental arc but I have no time for more essays today alas alas that adult life be so











