It didn’t take much to tip Rumpel into murdering the dark one and it takes a lot to tip a normal person into killing someone for the first time. Though that may just be the bad writing that is Desperate Souls.
Rumple was desperate and had no other options when he killed the dark one. It was Bae’s birthday and the soldiers were on their way to take him away. Rumple had already been through an ogre war so he knew what it was like and didn’t want the same fate for Bae.