You mentioned you had been trying to rehome another drake since you got him?
This is not the time to be trying to sell them, put a post on craigslist giving them away and I bet you they'll be gone in a day. I would be honest that they are agressive, someone else might not have any issue culling them and having a free meal and you certainly don't want someone with kids or something thinking these are suitable pets.
Personally, I would never waste them since I would have put the money into feeding them and everything. I think every poultry owner should know how to cull a bird, if you have any decent amount of birds at one point or another you will encounter one that needs to be put down. It took me about 6 months to be comfortable doing it and now I can do it no problem. It's supposed to be hard for you to do, if it wasn't I'd worry. But you've got two nice duck dinners for your family there, why foot the bill for someone else to have free dinner?