he told him that becuz he knew sasuke would need naruto in the future to stop tobi.or he was telling him that becuz if he killed naruto an the fox was revived tobi would come an get it an if the nine-tails end up in the wrong hands it could mean trouble even 4 sasuke.But he was just taunting him. What else could the reason be? Kurama was acting confident and was basically being a smart-ass by telling him he'd regret it. The fox was so confident that he freely told Sasuke the truth that it would be a bad idea. If Sasuke went against him and killed Naruto, the fox would appreciate it. Also keep in mind that the fox was astounded to see sasuke enter Naruto's subconscious. I think it was more a moment of honesty from the fox if anything, but you could consider it reverse psychology.