If you insult someone in the knowledge that they're going to attack you because of it and you intend to kill them when they attack you, that stops being Self Defence or even Manslaughter and becomes Premeditated Murder, pure and simple. You went into the situation with the intent to kill that person. Psychologically manipulating them into a place of vulnerabilty for you to strick only makes the crime more heinous.
At the time of the attack, he wasn't armed, he was simply big. Following the letter of the law, lethal force was quite arguably unjustified.