Hi, new Yr13 here! I did an EPQ this year and am awaiting my results (1 week to go - surprisingly not nervous lol) so hopefully I can help! It sounds like a really interesting topic, though obviously you have to be careful not to include anything that is on your specification (I think it just has to be something you don't cover at A-level, so I'd check with your teacher if you're planning on doing History, and maybe Psychology if you're looking at the workings of his mind).
I'll give some general tips for creating a question and then try to suggest one:
-If you're planning on doing an essay (afraid I can't speak for an artefact as I did an essay) then your question needs some kind of argument that you can evaluate. That being said, apparently it's best to avoid use of "to what extent" as it's difficult to fully determine the extent of something in 4000-5000 words.
-Be as specific as possible - try and keep a pretty narrow focus to avoid going off on a tangent. I had to change my question (looking at the impact of the lobotomy) as my original question was too broad to cover in my essay.
-Make sure it's something you're interested in BUT is also answerable. Fortunately this was not an issue for be but be aware you may find after you've done all the planning that there aren't enough resources available to back up your argument.
-I guess the only other thing is to make sure it is appropriate. I'm sure you won't be trying to suggest anything ridiculous but your supervisor has to approve your title, and your topic is obviously quite a sensitive one. From what you've said, it should be fine, but just be aware of what you're arguing about or trying to express.
It's tricky, but here's a possible question: Is there a biological cause behind Hitler's tyranny?
I wouldn't worry about creating a title too much at this stage, you'll do work on this in September/October to plan a project based on your interests and looking at how to create a question. If you already have an idea of what you want to do, you're probably already ahead of your classmates! Regardless, I hope it goes well