The dagger does not truly kill the dark one. Think of the dark one as a parasite. It latches onto a host and keeps the host alive and immortal so that it itself can survive. Whenever its host is stabbed with the dagger, the dark one latches itself onto the person who did the stabbing thus gaining a new host. The host cannot stab themselves with the dagger because they are just putting the dark one back into themselves and thus surviving.
True love’s kiss- I think this may actually work in breaking the curse. If the dark one parasite infects its host with evil then it is likely going to be defeated by something as pure and good as true love. In general the likelihood of anyone loving the dark one is pretty slim. Then along comes Belle and she sees Rumple, not as the dark one, but for the man he is underneath. She truly loves him and he loves her and that powerful emotion is enough to threaten the dark one’s hold on Rumple.
It could explain why Rumple let Bae go, the son he loves more then anything. The dark one parasite sensed the threat the portal held for it. It refused to let Rumple go with Bae. When the portal was gone, the dark one wasn’t threatened anymore. Notice how Rumple went from “I can’t” when the portal was there to “I want to go with you Bae” when the portal was gone.
All magic comes with a price!
Keeper of Felix