So, one of my son's friends drew a picture of him using a shotgun to blow off my son's head. Yep, charming. They were friends! I mentioned it to the teacher who said to the whole class without singling anyone out that pictures of violence or using weapons on someone are not appropriate or allowed. Done.
I don't know what is going on but in this day and age, I would not assume psychopath. Really. He may have an older brother that plays video games or saw something in a movie and it wasn't personal to your daughter. Crazy as that sounds it happens. We have another fiend who was playing Call of Duty with his dad at that age. I'm not minimizing the picture but as a mom of boys, this stuff is not foreign territory to me. I have three big bins of nerf guns and nerf swords and axes. How I became 'that' mom, I have no idea but this is what boys love to do. (well, a lot of them). I never was going to have gun toys in my house. But my son at 3 took something that was supposed to be a power drill from his toy tool chest and went around saying he was a policeman and used the toy power drill as his gun that he holstered in his waste band. Boys!
The boy asking for a knife is a call of attention to himself. Psychopaths don't ask for a knife. They know how to get one. He was feeling low and wanted people to know. (most likely). Things probably went on behind the scenes that you don't know about with regards to the school and him. For his privacy, they won't share that with you. And they aren't going to keep a child out of school for a drawing. We had a boy actually DO something violent and he had in school suspension for a day. (he stabbed someone in the hand with a pencil).
The main problem here is if this is all making your daughter FEEL unsafe. I have no doubt that she is overall safe. But if she feels unsafe, that is hard. But also, don't project your feelings onto her. Meaning, kids feed off of us. Do more listening than talking about this with her. Get the school counselor involved who may have some good ideas. Ours was fantastic in elementary school. I also think helping people learn how to use their words is essential. No, I don't like that and stop. She can use those words when needed with authority in her voice for a life time.
Hope it all blows over! good luck