Random harmful acts?

So when I am doing something and most of the time its when I get bored or just Dont have my mind occupied with things to do or whatever I start to think, What if? I may be standing in line at the grocery store and I just think what would this person in front of me do if I just punched them in the head out of no where and I wonder what would people think but just as quick as I have this thought I think about all the negative things and dismiss the idea. What worries me is I have these same type of thoughts but about killing instead. For example I was helping my dad work on a car and I thought what would he say or think if I just stuck a screwdriver threw his neck. Then after that thought I start to think about jail time, losing my job, not having free choices or enjoying life and everything in it and how much I love my dad and I move onto normal thoughts nothing to do with harm.

I don't have these thoughts everyday or even weekly it might be once a month or less but mymind is also telling me this seems odd if I didn't have any intent to do it why would I have those thoughts and I Dont want them to grow into reality 20 years from now. I have never thought about mass killing its just always some random single person that I may be interacting with or close by to me. Does anyone else ever think about doing something so random and crazy just to see what people would say or think and make them scratch their head and just ask why? When I see documentary's about killers in jail I try to understand why they would do something so terrible out of no where and I just can't wrap my head around it.