No, you don't. What stops people is actually access to knowledge. There are killers of varying levels of efficacy; making killing easier means more people die. It's 1-1.
How many people have shot up electrical substations, a fairly trivial way to causing widespread harm that was popularized in the news just a few years ago.
But it's only happened a couple times. Because most people just can't be assed to harm a hundred thousand others.
Most people don’t want to hurt lots of other people in an untargeted way, yes.
But there are a lot of people, and, apparently a few of them shot up some electrical substations?
If you hold constant the (very small) fraction of people who want to cause mass harm, and increase the average ease with which a randomly selected person could cause mass harm , you would expect the amount of deliberate mass harm to increase, even if the fraction of people who would want to do that is like, 0.000001 of people?
The kind of person who wants to cause mass harm is the kind of person who's thought about it for more than 15 minutes. If you think about it for more than 15 minutes, I'm sure you can come up with plenty of ways. (Even 5 minutes is probably long enough to produce 3 or 4 workable suggestions.)
If AI causes people to kill who otherwise wouldn't have, I expect it to be by causing and exacerbating mental health problems, not by providing knowledge. (See e.g. https://www.bbc.co.uk/news/technology-67012224, where a roleplay escalated into a concrete plan to kill Queen Elizabeth II over the course of two weeks. The plan wasn't, however, a very good one.)
5-8% of all mass casualty perpetrators are a combat veteran, between 92-95% of all mass casualty perpetrators are not combat veterans
By far, there are more veteran suicides than there are mass casualties caused by combat veterans. Veterans are more likely to kill themselves than to kill you.
DC sniper was 24 YEARS ago, prior to the invasion of Iraq during the GWOT. Not only that, John Allen Muhammad didn't even see combat, his job was to dismantle old rockets! He was a mechanic and truck driver!
The point of discussion is: "What stops people is actually access to knowledge."
Statistics show this to be clearly false: Knowledge is not required to be effective in causing a mass casualty event, in fact, lack of formal training is a higher predictor!
But it's only happened a couple times. Because most people just can't be assed to harm a hundred thousand others.