Mad3: Minc is a good character.
Sure, I don't think anyone's disagreeing with that. The point is that is being made is that slaughtering his party in a homicidal killing-spree because of a disagreement is
seriously out-of-character for him, and anyone of the good-alignment in general.
If Minsc actually had zero control over when and how he rages that might be different, but this is not how he's portrayed in game; he's shown to have the ability to influence his rage emotionally, wielding it as a weapon in battle. Due to the fact that he has a demonstrable level of control, the "temporary insanity" theory doesn't help since it just results in a catch-22. If Minsc is not culpable for his actions on any level then he's true neutral by definition (put bluntly, if you're not responsible for evil acts you commit then you're also not responsible for good acts you commit). If we accept that, on some level, Minsc is a moral agent, then he bears responsible for failing to control his violent tendencies.
Now, as I said, throughout the vast majority of the series Minsc keeps it together. He
doesn't go nuts and go on a homicidal killing-spree, he
does manage to pull himself together and avoid committing acts he'd regret later. That's why he's chaotic good; whatever violent tendencies he has, he controls them and makes every effort to ensure there isn't innocent blood on his hands. That's exactly why this outburst is so abnormal; it's completely contradictory to his moral ethos.
Mad3: If his bahaviour is defined by the 3 rules above and he wants to fight evil, doesn´t this make him lawful good?
Being chaotic doesn't mean you have to be inconsistent. What it does mean is that you don't allow societal expectations or customs to guide your behavior. Lawful characters are conformists who structure their life around their place within a social hierarchy, while chaotic characters are individualists who often chafe at the expectation of "fitting in" to that hierarchy. Minsc doesn't care what other people may think, he seeks out evil and punches it in the face.
So there is only one question left: Why is he a ranger?
Because a character class describes what abilities you have, not who you are. Minsc has the abilities of a Ranger with the addition of a custom berserk ability. You could argue that maybe he'd be better-described as a berserker kit fighter, but he doesn't need to be.