For one, Picard wasn't killed and then his parts used to create a new being. In my mind, after the patterns of Neelix and Tuvok failed to rematerialize and were lost, they were dead. If someone finds a way to restore them, great, I'll try it, but not at the expense of the new sentient life that was created. Furthermore, after weeks passed, they SHOULD have been declared dead, if not lost like all other crewman whose signals have been lost over the past couple hundred years. To me, that is the main difference between what happens to assimilated crewmen and those lost in transporter accidents. Assimilated being were never really killed, they were brainwashed and enslaved. However, after all the years that Annika was gone, I would consider her dead as she was more Borg than human. On the flip side, I would want a court martial held for Picard due to his action in FC killing a number of his own crewman, especially the ones who weren't fully assimilated yet.