Dr. Dre is making his most powerful confession/apology for attacking journalist Dee Barnes back in the '90s, and imploring other men to keep their hands off women.

Dre faced the "dark cloud that follows" him during part 2 of HBO's documentary "The Defiant Ones." While the doc focuses on his success in the music biz, Dre admits he absolutely lost it on Barnes in 1991 at an L.A. nightclub.

He says, "I was out of my f**king mind at the time. I f**ked up. It's a major blemish on who I am as a man."

He's apologized publicly for the beating before, but to be honest -- that came right before "Straight Outta Compton" was released ... and many saw it as a PR move.

This one -- which includes an intense warning about domestic violence -- seems genuine.