Although the Blood Knight is a simple concept, as the characters in question are just people who want to fight for fun, in reality this is far more complex than it seems. Hence a scale,
The Mindset of the Blood Knight...
The Boisterous Bruiser kind: A person under this kind of mind set sees any opportunity to engage in a combat as equivalent to throwing a party. They see it as fun, exciting, and something everyone can participate in. Examples: Hercules. These are very rarely villainous, and tend to be good guy kinds.
The Spirited Competitor kind: They are up for any kind of challenge without question. They do not enjoy hurting their opponent, but they certainly enjoy the battle as a personal challenge. They are usually more fair and sporty about their challenges, and are generally the least violent kind amongst these four kinds of Blood Knight. Examples: Ryu, Goku. These tend to be heroes.
The Unfettered kind: One who has not only devoted his life to the fight, but who is bloodthirsty and is not above amoral acts to get what he wants. However, he usually has some kind of moral code or respect for his opponents. He will probably not fight someone who doesn't want to fight him (because it will not pose a challenge) and will have enough self-control to not kill indiscriminately, but he will probably not have issues with killing (it is part of fighting, after all) and neither he will have any compulsions against fighting anyone no matter where he stands (ally or antagonist). This kind's most noticeable trait, still, is putting his love of fighting above matters of right and wrong. Examples: Akuma, Kempachi, Zodd. In terms of alignment, these tend to be antiheroes or villains.
The Ax-Crazy kind: It says it all. The Ax Crazy Blood Knight has sacrificed their morality and sanity for the sake of battle and bloodlust. He will most likely kill anyone who stands in his way, not care about collateral damage, and generally be a berserker of the worst sort. You are very much not safe next to them, needless to say. Examples: Khorne and the vast majority of his followers (although he started up as a type 3, and still has some parts of a type 3), Gauron. These are almost exclusively all evil, with Chaotic Evil being the next-to-canonical alignment for them. Only the darkest of antiheroes and the most bloodthirsty of villains happen here.
This might be the result of losing everything the blood knight cared about, so that he can't fight FOR someone anymore, but his only reason left to live is to fight AGAINST someone. Because of this, nothing to him is more important than killing his enemy. (One good example of this is the real life Baron von Ungern Sternberg during the Russian civil war.)