I don't see how Nash used the Raptors as all media reports were that the team approched him with the big offer. He also did not know whether the Lakers would take him on board when he spoke to them.
Free agency is just that, free, and the player is free to talk to any team about playing for them. Colangelo's gambit in tying up the Knicks with the Fields offer sheet would only have been effective if they were the only other team with cap space for Nash.
I did read a quote from Nash a few days ago where he talked about wanting to play for a serious contender and he did say something to the effect that the Raptors were not quite there yet. He listened to their offer, and presumably those of the Knicks and Lakers, and selected the one he liked the best. Had he said he would come here and then backed out a couple of weeks later, we could rightly feel betrayed, but not in this case.
Another aspect to this is that Sessions unwisely opted out of the last year of his deal and opened a spot for Nash on the Lakers. The starting point guard spot was available and there is a decent backup in Blake, who can play 15 - 20 minutes a game, and start some games if Nash needs a maintenance night off. Blake will likely back up Nash, the same role he had when Sessions, and previously Fish, were starting. Here, there would have immediately been the Calderon - Nash issue, and Nash is a consummate team guy who may not have wanted to make those kind of waves.