^ I was agreeing with you. I added the detailed account to give a more focused context of the discussion. Laying out all the pieces, we can see the whole picture clearly. So people can reflect on everything that led up to Kirk's decision to fire on the Narada. The fact that it was Kirk's first day was not all that led up to his decision. Nero's actions drove Kirk to a reaction. Which the OP found "petty and vicious". Kirk killing Nero can be descirbed as Justifiable Homicide: a homicide (as by accident, by misadventure, in self-defense, in performing a legal duty like quelling a mob or carrying out a death sentence, in preventing a felony involving great bodily harm, or in defense of one's home or members of one's family) justified or excused by law for which no criminal punishment is imposed.
Justified since Kirk did it in the defense of Earth, and the Federation planets.
**It's similar to the controversy surround MOS, with Superman killing Zod. With everything we saw in the film up to that point where Superman has Zod in a headlock. A lot of people feel Supes crossed the line by breaking Zod's neck. Here again you can call it Justifiable Homicide by Superman in the defense of Earth.