^^Many people have already explained the need for Tris' death numerous times. If you put it that way, there's no need for her to live either. Both ends can be executed pretty well. And no, it's not most fans, from my experience the number of both sides is equal- some are unahppy with the event within the story, but understand making us unhappy was the exact function of the event, as well as all the reasons you seem not to pick up. Some are even not unhappy within the story, both cases are actually moderately common.
To repeat, it was supposed to make us sad, if it did VR did a good job for what she wanted to do. Why? Because that's what this story is like (its genre really is more that than a fairy tale, but some newer works tried to corrupt that), and because it's for deep messages, not just pleasure. Rather than get upset and scream at other users about that, we should understand the nature of this work andd that it's exactly what the author wanted, we don't get to decide how it ends and if we don't like it, there's fanfiction or other works. I mean, things like being 'scarred for life' are definitely exagerrated for an impression about a work of fiction, where things aren't real, and when you haven't even been through a quarter of your life. It's something that feels very strong now, but definitely not for life, or at least not in a bad way- if we're going to feel this way, we should not read or watch things at all, one of their function is to cause many emotions, not all of which are pleasant.