Stalkers do what they do for all sorts of reasons: anger, spite, frustration, mental health issues and so on.
Substance abuse and personality disorders are two common factors often found in stalkers.
They can harm others with absolutely no sense of remorse or guilt.
They spend their lives learning how to mimic normal human emotions, but they don’t actually experience any of those feelings.
Or maybe they’re just the totally normal guy who served you coffee this morning.