I think it also misses a special case, where a active shooting would have happened, but a 'good guy with a gun' stopped it before a death toll occurred by either holding the shooter at gunpoint or shooting them.
This would likely be a rare case that would be much harder to quantify but you know it will be argued it's needed for that case.
I'd argue Hanlon's razor is not a very good heuristic. It ultimately presupposes the user of it is the mental superior in the situation, and does not take into account polarized and ambiguous controversies. It also encourages energy wasting by presupposing the issue lies with mental capacity or education, suggesting that you could educate your opponent out of their stance.
I'd recommend moving towards more energy-conserving practices. Rather than arguing your points directly, it's better to first understand why the opposition would be taking their current stance and adjust your argument based on what common ground you both share.
Possibly the greatest skill is to just learn when it's no longer worth your time to argue with them.