Why do many Muslims claim that irreligious people who leave their religion are psychopaths?

It helps justify them in 🔫ing apostates, which in turn deters people from openly ditching the faith.

Why is this the first time I ever heard of this?

If you're a psychopath, you should become a Muslim because then a lot of things that would be actually be illegal would be encouraged by your religion. Killing other "sinners who will not pray to Allah", having 4 wives, beating your wife, whipping yourself with knife-edged chains, etc.