Ex-Kony Soldier -- Warlord May Have CURSED Jason Russell

African warlord Joseph Kony is a master of voodoo ... who may have willed an evil spirit to poison the mind of Invisible Children co-founder Jason Russell ... this according to a former member of Kony's child army.

TMZ spoke with Richard Akena ... who was captured by Kony's forces 4 separate times when he was a child and was forced to carry out Kony's evil bidding.

Akena tells us ... most people who were captured by Kony believe he is "being controlled by a devil spirit."

Akena says it wasn't unusual to see Kony try to use voodoo magic in an effort to thwart his enemies in Africa ... and he believes the same voodoo magic could explain Russell's naked meltdown .

Russell's family says the breakdown can be chalked up to "reactive psychosis" ... caused by stress and lack of sleep.