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Look up 'amphetamine psychosis' and 'dopamine hypothesis of psychosis/schizophrenia' to see how extended use of amphetamine can change a person psychologically. Becoming violent is only the tip of the iceberg of everything it can cause to happen to a person emotionally.
The reason he can't or won't feel like doing anything without it is because amphetamines cause a massive release of dopamine (which is the chemical that provides us with feelings of desire/want/motivation) when used. But then leaves the brain incapable of producing enough dopamine as it should in the wake of that artificially boosted production. So more amphetamine has to be taken to feel ok or to have any motivation again... very quick cycle into addiction.