What turned big boss evil

Answer depends whether your talking about the original big boss or the female big boss from MGS 3, if u are talking about her then play the game and watch the cutscenes, it will tell you she did it to end the cold war, she was a hero


First Of All if The person up there had Truly played the game then they would know that the BOSS not ( female big boss) died as a true patriot and was nothing close to evil.
*she was betrayed by her own country*
As for Big Boss, he was never evil , He was trying to fulfill what the Boss couldn't finish.