seriousonly please, thank you.
when not why

to refresh his character

l think SuperBrawl 1992

Edit: He also turned heel in 1999 when he allied himself with nWo.

He turned heel at Halloween Havoc 99 when he hit a babyface Hulk Hogan with a bat. The heel turn didnt last long and two months later Sting was once again a babyface

It was at Fall Brawl 1999 when he decided to hit Hulk Hogan with the bat at the end of their match.

Yes Fall Brawl in 1999.