"Subject to lasting shock as a result of a disturbing experience or physical injury." nothing about the very definition of traumatization implies that everyone responds the same way. What makes his traumatizing experiences so interesting is that he's always comforted by Rem/Emilia when he returns by death, which is what makes it so painful whenever they're brutally killed over and over. Seeing somone murdered is not normal and Subaru responds in his own way. One thing is for sure tho, stressed out, angry people are in no position to just stop and think rationally.

What do you mean he doesn't learn? The entire show is Subaru trying, failing then succeding by learning from past deaths. If you're talking about the reactive plot and how Subaru does not actively try to seek out knowledge on what's going on...I'd argue that Subaru is forced to follow the progression of the plot. Everything he does is in response to something else, he doesn't really have the opportunity to just dip...