My 5 year old has been drawing explosions and blood etc for a few years now. He hasn't experienced any trauma, is a happy child and has not been abused in any way. Some children have fascinations with things they can not understand completely.
There are some schools of thought that link the violence (and sometimes colours used) to traumatic events and recommend that things are looked into. This could be worth considering.
Sometimes children draw things that they know illicit reactions from adults - to get more attention in school etc.
If his behaviour apart from the drawings shows no indication of trauma, depression, or "acting out", then you probably have nothing to worry about, and possibly need to ignore the violent drawings in favour of drawing attention to any he does that are not violent.
You might also keep in mind that Halloween is just around the corner and what he drew is (I think) a pretty typical Halloweenie kind of thing. So I think I would go with what Araldia is saying - at least for this picture.
Did you ask him why he drew the picture ?
My first thought was that it will be Halloween soon too. There are graves and monsters everywhere right now. Keep an eye on him and try not to worry too much. Children are tougher than they seem.
He has a lot of other fears such as changing in public places.Has to lock doors and has to check locks.Today I got told he threatened to take a knife into school to stab a parent.....
And who told you this? Kind of makes a difference. Also, and perhaps more importantly, what parent? Why should a parent at school be in a position to cause a kid so much concern that they want to stab them?
Another thought is that there is a really neat set of books which help kids express their anger (in more appropriate ways). Its the "Way I Feel" series and has titles like "When I Feel Angry". You can find them here - http://www.amazon.com/When-Feel-Angry-Way-Books/dp/0807588970/ref=pd_sim_b_2
And you can find other titles by scrolling down the page. You might want to buy some and read and practice the ideas with him.