Picard. Data downloading his memories to B-4 was the equivalent of backing up a hard drive. Not a very sensient thing to do. Androids do not have souls (unlike Vulcans and their katras) so the memories are just files without the positronic net. After all he had fought for to be accepted as a sentient lifeform (TNG: The Measure of a Man) it was cheap to have Data extinguihed. B-4 was a cheap ret-con plot device. Now if Shinzon had reassembled Lore and teamed up with him, that would have made more sense. It still wouldn't have helped the film much though with all of its issues.