If you enjoy reading them then go ahead. If you start feeling a sense of conviction that you shouldn't be reading them, stop reading them.
Some things are clearly forbidden in Scripture. Some other things aren't explicitly mentioned in Scripture but it doesn't take a genius to figure out they aren't good (e.g. watching porn online - not explicitly mentioned but what Jesus said about lust was pretty clear). I don't honestly thing reading a murder mystery novel fits into either of these categories.
Sometimes people will get a specific conviction that they shouldn't be doing something, or should avoid specific situations. This isn't because of a universal Scriptural mandate but because for some reason it's likely to be harmful to their Christian walk, even if it isn't harmful to anyone else. For example, in general there's nothing wrong with having a beer but if someone is a recovering alcoholic it's not a good idea, and if someone is prone to get drunk on a small amount of alcohol and behave in a way that they are subsequently ashamed of it's best if they avoid alcohol. If that applies to you, chances are you will either feel a sense of conviction that you really shouldn't be doing this, or you'll become aware that doing it draws you away from Christ and you'd rather not deal with that.