Randy Orton would have been PERFECT for the Attitude Era. While some seem a little hesitant to say he would have been a huge superstar, I am not.
I think, if he were exactly what he is now, he would have been their biggest heel. Mankind was a great heel, but wasn't mainstream enough to be the top heel. Undertaker was evil, but he had been a solid face for years prior, so that hurt him a bit in my opinion. Kane was good too, but was really only meant to go after Taker, so he didn't really have the range Orton would have had. Rock was a great heel, but he was a tweener (like Austin) even when he was doing terrible things to poor Mick Foley. Big Show wasn't good enough in the ring. Triple H would have been Orton's biggest competition in terms of being the top heel, in my opinion. While Orton performs horrible cruel acts, Triple H was a bit more timid. He cheated most of the time, and relied on others to help him win most of the time. They would have very different, and I believe they would have been the top two heels in all of wrestling.
Not even during the Attitude Era could Orton have pulled off a tweener role. He is a pure evil heel. I agree that he would have had much more freedom to do whatever he needed to do to get heat back in the Attitude Era. While Austin and Rock were tweeners, they weren't evil. Austin was mean, but wasn't evil. Most of the time, post-Wrestlemania 14, he was doing mean things that we all wanted to do. The Rock did evil things, but was too comical do ever really be thought of as an evil person. I think the cruel punishment Orton would have put guys like Austin and the Rock through would have been too much for the fans to cheer. His off-balance, nearly psychotic character would have been amazing to watch back then.
P.S. Brian Pillman was off balance and mean at times, but wasn't sadistic. I don't think it's a good comparison.