Lore was deactivated and disassembled. They'd found him like that as well. Data didn't say 'disassembled to his component parts,' just disassembled. It could just be that he intended to have Lore kept offline until someone else understood positronic brains and would be able to help to make him more sane (whether that would even be possible, your guess is as good as mine).
Either way, though, IF Data did 'kill' Lore, I believe the Federation would accept it, and not just for reasons of 'Data's the only android we've declared sentient.' Remember Worf killed Duras under Klingon law and received no punishment for the deed. I believe that Data would have similar protection under Federation law, even if he had no personal laws for his 'species' - when you're a species of one, you don't really make laws, just personal judgment calls. Not to mention, he IS Lore's next of kin, so he'd be the only one in a position to complain about Lore's treatment in the first place.
Here's another question to ponder -- after Data (presumably) disassembles Lore, where did Lore go? Did Data keep his parts in storage on the Enterprise-D, and if so, did Lore's parts survive the crash in Generations?
One of the TNG novels, 'Immortal Coil,' said that Data kept the 'bodies' of Lore, Lal, and a few prototypes he'd discovered in a sort of 'family plot' in one of the Enterprise
science labs, and that they'd survived the crash of the D and were moved to the E. I don't know if any other novel has worked with this fact, or what would have happened to it after his death, though.