Your love interest has been taken away by your mortal enemy. AGAIN. How would you release that anger. If I were Mario, go out and kick the butts of the culprit's troops, and save my princess. That anger is more out of the repetitive nature of that rivalry. You know, Bowser kidnaps the princess, Mario goes and kicks Bowser's butt, and saves the princess.
Also, I saw those kills as means of protection. Most of those creatures, except in the RPGs are Bowser's troops, and you know how Bowser tends to hold grudges. Those troops are out to kill you.
But yeah, I think sometimes that hat traps some of the steam in our hero's head, and that might give him a hard time of listening to reason.
Are you sure Mario's not just teasing Luigi at the end of MPT? Big brothers tend to tease. Luigi's expression; who likes to be teased? Luigi's probably getting tired of his big brother's teasings, no matter how lighthearted they were meant to be. But ignoring your brother's adventures; that's just rude.
Yeah, sometimes, Mario can be a little rude in the games, but I don't let all those different games change my view of his personality. After all, they are just games.