Nash wasn't definitely going, whereas Hall was. Nash didn't want to go back to WCW, because of how they treated him when he was there previously, with the stupid gimmicks and whatnot. Then the whole "Motherfucker, this isn't about you" story sealed it.
Hall talking about it:
http://www.wrestlinginc.com/wi/news...kevin-nash-frustration-after-bret-hart-match/
Hall believed that Nash's decision to leave WWE in favor of WCW came following In Your House 6: Rage In The Cage, where Diesel faced Hart in a steel cage match. Hall stated that the planned finish of the match was that the
Undertaker would come up through the bottom of the ring and attack Diesel after Diesel hit Hart with the Jackknife Powerbomb. Essentially, Diesel would have the match won, but outside interference would allow Hart to retain his championship.
"Bret won't take the powerbomb, so now Kev comes back and goes, 'screw it. I'm out of here' because he's saying, 'wait a minute - this is what they're doing with me? When it comes down to a decision this guy booboo faces and they don't do it?'. So he goes, 'I'm out of here'," Hall added, "when it really comes down to it, the guy coming in with the jetpack wins. I mean, yeah, it's one of those moments where you go, 'aha!'. I mean, I don't get it. I've never been a guy to b---h about a finish, so I don't know. But, that's what happened. It worked out great for me because I knew at least I have one friend there [in WCW]."
Nash talking about it: