Belle was okay with the would-be murder of Pan so I don’t know why she would be so up in arms over Zelena’s death especially as Zelena put everyone through hell for a whole year. Whenever Belle has asked Rumple not to hurt someone, it’s always been on his behalf rather than the other person. So I could see Belle being quite angry and disappointed in Rumple for giving into his hate and need for revenge and for lying to her, but I don’t think she’d be all that bothered by Zelena’s death.
1) different situations. Zelena was already defeated. “Pan” was taken over the line so that Emma could question what happened to Henry. Pan was only going to die after Emma was satisfied that nothing was wrong with Henry. Zelena was in jail and everyone was safe.
2) Belle even tells everyone that she could have curbed Rumple’s homicidal tendencies if she suspected him of going after Zelena. She is not okay with even the idea of him killing Zelena. The problem is that she trusted him too much.
3) We actually have very little evidence for the whole “Zelena made their lives hell for a year.” While they were in the EF, we never saw Zelena actually do anything other than fly in through windows and occasionally threaten Snow and the baby and have a sass off with Regina. No one was being harmed, so far as we know. you’re making conjecture.