That's an interesting question, gbidari. I remember reading somewhere that Kasparov said chess was the most violent sport there is (sorry, can't cite). I wouldn't think that steroids would do any good in a chess match as they inprove physical performance. And yes, they do make one more aggressive. The question then, do they improve mental performance? I don't know of any studies that confirm the idea, but that's not to say they don't.
What's "killer instinct"? The automatic response when perceiving weakness to pounce and destroy? I would think that is essential to master chess.
I also play poker (NL Texas Hold 'em). Aggression is a big factor there as is with almost all competition. So, to answer you question, I don't know. But you've given me something to really think about and do some research on. Thanks!
Former World Champion Boris Spassky is a very cordial and classy fellow, so on some level it was a bit surprising when he said the most important thing for a chess player to bring to a tournament is the desire to want to kill everyone. He was talking about that term we often hear in sports called "the killer instinct." I once read that a doctor suggested to Nigel Short that he get testosterone injections for his match with Kasparov because he thought Short was too nice and that a little aggression enhancer would help him. Short declined, and later got crushed in his match with Kasparov. Maybe he should have taken the shots. Since a killer instinct is so important to chess success shouldn't our first priority be to try and bring it out? Or is the killer instinct just something you either have or you don't?