Thoughts like this are not unusual. Don't be alarmed that you have them. For me it's a reaction to stress. I've had some terrible thoughts, but that's all they are.
You could be suffering from depression. When I'm depressed I start to plot out ways to commit suicide. If this is the case with you, you might want to talk to your doctor about it.
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People are strange, when you're a stranger
Faces look ugly when you're alone.
Morrison/Krieger