Could a conscience or moral values stop someone from carrying out a mass shooting? Sure
Could difficulty in getting a gun stop someone from comitting a mass shooting? Also possible.
What about fear or shame? Again, also possible
There is quite a difference between saying you are going to burn your ex's house down and preparing to do so but backing out at the last second. If you poured gas on her house but it was too windy for the lighter to ignite the fire and you gave up I don't think that would be in the same vein as mentally saying you are going to do it but not taking any action towards doing so. Certainly I would not lump just any high school student who felt like going on a shooting rampage as someone who almost went on a school shooting.
There are people who go to work every day, planning to rob the bank where they work, or kidnap and rape the hot receptionist in their office, or even shoot up the building. They draw maps, devise plans, buy supplies, read books and websites on "how to" do their desired crimes, and so on.
You know why almost all of them never do? In large part because the simple process of fantasizing, planning and dreaming of their perfect crimes being executed are, in and of themselves, therapeutic. They know what they are dreaming of is wrong, but they still want to think about it -- visualize themselves doing it -- and that can create a release of all the pent up anger, frustration, fear or whatever other emotions caused them to think of doing this criminal act.
So, perhaps your "almost shooter' went through all those preps to commit the shooting then decided against because the prepping helped calm his anger. Sometimes just knowing you could do something and chose not to is just as satisfying as actually doing it and having to suffer the consequences -- or worse, see the impact your actions had on innocent people and their families and friends.
You want to believe he was a mass shooter up to the point he chose not to. What if he was never a mass shooter, but was enjoying the thought of being one -- just without any intention of ever going through with it?