While he was a pirate Killian had a fierce reputation, known as a dangerous and fearsome Captain during his pirate years. Killian was a tormentor, at times. He enjoyed bullying others and putting them in their place for the sheer purpose of entertainment. He has a strong sense of humor and came come off as very cocky or arrogant at times. Despite his rank as a pirate, he speaks with eloquence and intelligence, which shows that he has a high education than one would assume with a common pirate. He is arrogant and selfish sometimes but it is common, he is patient with certain things and he respects only if he is respected. Killian is a man of honor and never steps down from a challenge or admits fear and defeat. He is not trustworthy and was never been known for keeping promises or even secrets. Killian can be ruthless at times and has no regrets for his choices or decisions. He takes pride in his achievements and rewards himself with the riches he loots from others.
When Killian became a vampire it was like something ignited inside of him. Killian's reputation proceeded him even though he was a vampire, his human traits became significally heightened. Along with the increasingly dark nature of being a vampire, he was unperdictable and self-destructive, he was even more ruthless at times and showed no mercy on the ones that begged and offered everything they had. Since his basic human traits were heightened his attitude was even worse, he became more reckless while putting himself in duels and challenges with others. He became impulsive and if it wasn't for Emma Fell he would have lost everything, including her. When she comfronted him about his behavior and stated that she feared for him
he simply replied "I'm embracing who I really am.."
After his reign of being a pirate was over Emma realized that it was time for him to change. Killian considered it and after looking back at all the innocents he's killed with regret he agreed. He became more caring, considerate, kind, selfless, protective, loyal, compassionate, more loving, affectionate, even more mysterious, polite, respectful, and even noble and heroic. Since Killian embraced his true self in the beginning it became easy to be in control of his instincts with his dark side. He has complete control over his blood lust and never allows it to get out of hand.