How can anyone not return someone's money?

I don't understand the mind of a scammer that owes you money. Meanwhile I'm very punctual (well my parents are the same way too) and will always return someone's money, my ex boyfriend scammed me and kept my money. I wonder how can someone like that even sleep well at night?

I would be feeling like garbage and my own guilt would force me to return their money.
How can anyone not return someone's money?
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