One of the required symptoms for being diagnosed with Schizophrenia is mania.

We can enter a state where we don't care about ourselves and may do stupid shit like drive way too fast or ride a skateboard down a mountain.

You still have your logic and morals in this mind state, it's more about self harming/ not caring if you die, but may put others in danger.

SSRI's are way worse. You don't even feel like you are you anymore, zero consequences matter, and your logic and morals go out the window.

Big Pharma causes more mass murder than mental illness.

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Not that that's out of the way, most serial killers do not feel remorse, but some do. They are addicts and in their mind they can't stop. Some will even kill themselves because they don't want to hurt people anymore.

People who commit one time killing sprees typically don't have any regrets, and justify their actions. Usually blaming the victims for pushing them over the edge. Again, not all of them, some are left traumatized for life.