It has more to do with the Spurs offensive predictability than that "killer instinct". The Spurs, prior to this year, was the most offensively predictable team in the league, probably ever. Dump the ball to Duncan, 4 guys stand around and watch. It takes a couple of games to figure them out, and it's very much like the George Karl Sonics, they are great in the regular season, but when teams have a 5/7 game series to figure them out, they are much easier to beat.
A huge part of the Spurs success this year during the playoffs is their unpredictability. Manu off the bench, him freelancing and creating in the 4th quarter, lesser reliance on Duncan on offense and Barry and Horry offering different dimensions off the bench, these are all things that catch other teams off-guard, and require them to watch tape to come up with ways to match it up.
Very rarely has there been a team in the history where you KNOW exactly what they are going to do, and they do it over and over and over again to win a championship. It's one of the main reasons Malone and Stockton never won a championship, it's also why Barkley with his park his butt in the paint offense never won anything. Pop has grown as a coach, and is more willing to try different things, this makes the Spurs a much better team.