I think people who have this intention are normally emotionally unstable in some way. This may be because something bad has happened to them to make them feel this way or it may be more deep rooted psychological problems which have made them mentally ill.
I think that sometimes people lose perspective (a sense of what’s important), morals (a sense of what’s right) and empathy (an inability to see something from someone else’s perspective). You’d have to lose all three!
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