Is it permissible to kill a Muslim if their future harm is highly probable and they cannot be deterred, such as if they were to steal one's money or reveal one's secret and harm them?

Unlawfully taking a life is one of the gravest sins, as Allah Almighty says: "And whoever kills a believer intentionally, his recompense is Hell, wherein he will abide eternally, and Allah has become angry with him and has cursed him and has prepared for him a great punishment." (Quran 4:93) Stealing money or divulging a secret does not justify killing the perpetrator; rather, they should be dealt with appropriately and punished as they deserve. The imposition of punishment rests with the ruler or their representative, not with individuals.