Well then you're losing a lot of support by killing the bystanders (not yours lol but y'know normal people with a conscience might not like it as evidenced by common perception of said IRA's actions) and a lot would have to go right for that (how close is he to the fence, is there a stage, can your sedan make it through all the bodies in between you and him, can his security take you out before you get him, he's a moving target so will he see the carnage and stand still or more likely will he run, will you be able to find him in a panicked crowd, etc). If the goal is to be an anti-hero it's best to avoid that and be extremely targeted with your violence. Luigi wouldn't be nearly as popular if he had happened to also shoot some random witness for example.

Furthermore since this was about "school" shooters not "mass," a large percentage of those children lack a civic. Some of them don't even have a license, and their disappearance as they drive 3 days across however many states to get to musk will be noticed, especially if they steal mommy's car too and pass any flock camera or ALPR, but also they'll likely be noticed by the parents themselves in all but the most neglectful households.

For some cults you can actually do that, but like, there's still Branch Davidians, mixing cults and Christianity is a very powerful tool. Just look at Mormons, cult - check, christian elements - check, one of the most powerful religions in the states. Some don't even need Christianity, see: Scientology. You kill David Muscavich today and another takes his place tomorrow, and you're not going to be able to kill all of them even if you are fine with killing their victims too due to some warped sense of justice.