I think you’re right that most likely the cursed dagger wouldn’t allow Rumple to die by his own hands. Interesting territory to play in, though.
I don’t know if I’d site Zoso’s actions are necessarily a flawless litmus test. Zoso definitely wanted to die. This is evident because he goaded Rumple into stabbing him. Why not take the dagger and stab himself? Well, possibly a combination of vindictiveness and also an attempt to give Rumple the tool he’d promised him. (He was also in Rumple’s control, and since Rumple had worked so hard to get the dagger, he wasn’t about to hand it over and let Zoso do whatever he liked with it.) The Duke may also have ordered him not to steal it, so he couldn’t retrieve it himself.
I think you’re right, mind you- I just think if one wanted, there are some other possibilities to explore in Zoso’s motivation in that scene.
"Nothing is easier than to denounce the evildoer; nothing is more difficult than to understand him."
