Can someone explain why Rumple had to kill himself in order to get rid of Pan?

I wondered that too...maybe it's because they're blood related and if he stabs anyone blood related to him "with his dagger" then he also dies. But I'm not sure.

Okay here's my guess (lol, yes I see this Frage now): Rumpel in Neverland send his shadow away from his body so he didn't have any. Pan didn't have a shadow either. So both of there bodies were "empty" to say it in some way. To kill Pan Du had to kill his shadow. So he called Pan's shadow into his own body. Then he stabbed himself with his super special dagger xD, killing both of them in the act. Killing the shadow he was killing Pan's youth, powers and eternal life, which turned him back into a normal person that could die getting stabbed. It wouldn't have worked if he had just stabbed Pan's body because his powers would remain intact in his shadow and he couldn't get killed as easily. Rumpel had to sacrifice himself. So I think that was it but I'm not sure:)