I can see it in their eyes—THEY'VE ALREADY DIED INSIDE.

I think old Kirk learned his lesson when Spock died in Wrath of Khan and basically rubbed Kirk's THERE IS NO SUCH THING AS A NO-WIN SCENARIO words in his face -- though I don't think he meant to do that, it must've made Spock's death hit Kirk even harder. In this world, though, he already has seen proof that things aren't always going to work out. Kirk Prime was more experienced and he had more self-confidence to back up all his bravado, but he also seemed lot more psychologically healthy than this kid.

BUT YEAH there are differences between the Kirk's. regann wrote up about the movie and pointed out the fact that nu!Kirk didn't witness the massacre on Tarsus IV -- I think an experience like that in Kirk Prime's timeline could make him far less eager to see anyone die needlessly.