As you say , it is a little hard to picture a 1400 year-old intellect not being more mature, but 2 other possible explanations have come to mind.Greybeard wrote:
It is therefore quite plausible that Misa's reaction to death is not unlike that of the typical 5-year-old. I'm frankly not exactly sure what that is, despite having had a much-loved relative die when I was about that age and having had my father die when my oldest son, his grandson, was about 6. However, I suspect that there is a strong element, not of denial, but of complete inability to comprehend. It wouldn't be quite the same with a 1400-year-old intellect (even one severely stunted emotionally by exactly the babying that Misa has been complaining about) superposed on it, but who knows what it would actually be?
One has to do with the fact that when one experiences a tragedy, it often takes a certain amount of time for our minds to process it before the reality of it sets in. Because of the scale of an elf's lifetime, their reactions may take more even more time, and so the devastation simply may not have had time to become real in Misa's mind.
The other unknown is Misa's religious beliefs. For all we know, she may belong to a sect that believes elves are truly immortal and that death is just a passage to another level of existence. One with a truly unshakable belief in such a system may be unaffected by deaths, even such unexpected ones.
This death is meaningless. To us true believers, time is just an illusion, albeit a stubborn one. - Albert Einstein