Last Sunday, while she was at Granny’s diner, Zelena overheard Grumpy’s announcement that Regina was preparing a memory potion which would restore their memories of the lost year in Enchanted Forest. So why is Zelena hanging out with Snow, instead of trying to foil Regina’s memory potion?
Possibly David saw something in EF that she wants to erase from his memories forever before he gets ahold of Regina’s potion. Or, to replant those memories with new memories?
It would be foolish to kill him, but why not just bring in the monkeys to take him?