According to ESPN's Brian Windhorst, the Lakers feel as though Leonard "played" them throughout the process, and led the team on even though he didn't intend to sign with them. While waiting on Leonard's decision, the Lakers missed out on the opportunity to sign other players to contracts in free agency.
The Lakers aren't alone in their discontent either, as the Raptors were also less than pleased with the way that Leonard went about his business this summer, according to Windhorst. Leonard ultimately signed with the Clippers after holding discussions with both the Lakers and Raptors, and both teams believed that they were in the running until the end.
From Windhorst (via the Big Lead):
"I've heard complaints in the days after the signing. I heard complaints from the Lakers that they got played. I heard complaints from the Raptors that Kawhi came in and asked for the sun, the moon, the stars then left them at the altar.
... I kind of think they were all still in the game, and they all felt like they were still in the game. If Toronto did not still feel like they were in the game, they would not have sent the plane for him, they would not have done the big presentation. If the Lakers didn't think that they were still in the game, they would not have delayed the Anthony Davis trade like they did to wait and see if they could get him."
Leonard certainly took his time in making his decision, but whether or not he purposely played the Lakers and Raptors in order to sabotage their backup free agency plans is not known. Both the Lakers and Raptors had to know that they were taking a major risk by delaying other free agency moves until Leonard made his decision, and ultimately the risk didn't pay off. However, that doesn't mean that Leonard intentionally wronged them.
At the end of the day there may be some hurt feelings, but all parties involved will have to move on, as the Lakers, Raptors, and Leonard's Clippers could all be in the mix for the 2020 NBA title next June.