If you've ever wanted to write a horror or thriller short story or novel, it probably involves some sort of killer in some way. I, for one, love to write about people murdering other people, which sounds really bad but its true. I love the gore and blood that comes with having a killer in one's story. The only hard part, though, is writing a good killer into your story. Anyone can write just anyone killing someone else, but not everyone can write it well.
First thing is first, pick how your killer will look and what they are going to use to successfully accomplish their many murders to come. Try to make it different and try to make your killer smart rather than the average horror movie cliche of having a stupid serial killer who doesn't even cover their steps from killing someone and easily gets caught. As for their weapon or way of killing people, you can choose something nice and simple like a gun or perhaps have them like to suffocate people, or you can go the more extreme route like me and choose the most bloody and horrifying way to kill someone and go off of that.
Next, try to give the killer something unusual that he or she keeps from their victims or something they like to find in their victims so they all have something in common and the killer can have a trophy of remembrance of the kill. Try to find something like that because it helps the reader understand the psychological psychopathic tendencies of your killer.
Then, you're going to want to then create a weakness for your killer. Every single killer has at least one weakness, no one is fully impenetrable. Pick something different though, something emotional helps tug at the readers heart strings and mind. If you choose something like "my killer's weakness is knives because they can kill them" then your killer is very boring and obvious. Obviously a knife will kill anyone! Choose something unique to your killer.
Finally, choose how you want the killer to end or prevail at the conclusion of your story or novel. Do they die? Do they live? What happens in the end? Jail? Freedom? There has to be something that happens to the killer whether they live or they die or else the story is boring and there are so many loose ends the readers won't enjoy the story anymore.
Now you have the basics for making a killer in your story and hopefully the killer you create isn't boring or typical like a lot of killers we see in books and movies today. Make your killer pop and be different so people read about him or her and they are shocked and intrigued with your story.