More often than not, experiences shape outlook. Someone at a young age sees their classmates murdered by someone with a gun? Of course the majority of people in that situation would be anti-gun afterwards. A few may not, but it is likely that those would have probably already been pro-gun to begin with.
It has always greatly irritated me that many in gun culture have been so quick to not only write-off most of these victims (less face it, all of the students in these scenarios were victims), but to even go as far to verbally slander them in various ways.
You never change someone's mind by calling them names, stupid, etc. You change someone's mind by listening to them, considering their input and life experiences, and at some point suggesting that maybe there is a better way/opinion. Of course it doesn't always work, but it sure does work better than calling someone names, telling them that they are stupid/wrong, and invalidating the fact that they survived a school shooting but conversely watched their classmates get killed. That never works.