Is it a problem if you constantly think of murder?

It depends on what you mean when you say that. If you mean you constantly think of murdering people, yes it is a problem and you need to see a professional psychiatrist before you end up hurting someone and spending the rest of your life in jail.

If you mean you constantly think of being murdered, you might have a problem if these thoughts really interfere with your life - you should see a psychiatrist for that too, as it's called a phobia and can be cured pretty easily. However, you might just be a very cautious person who thinks of lots of ways that life could be dangerous, so if it's not really interfering with yoru life, that might be it and it's no problem.

Of course, you might be a mystery author who constantly thinks of new murders for a book - in that case it's definitely not a problem. Just get back to your books!