#1, He never said that to Lore. #2, Data has done exactly that to a number of humanoid lifeforms, the most notable example he almost did it to being Fajo. He intended to kill Fajo (Rightly so imho) & he similarly deactivated Lore, which isn't killing btw. Even dismantling Lore is not a death sentence. They found him dismantled, remember? I always expected Data to fix Lore, once he had learned enough to create more Soong androids. Why does everybody assume this is the death of Lore? It may end the era of his horrific reanimation, but he is not destroyed
Plus, Data must certainly have considered himself sentient & worthy of the rights thereof. He was the sole supporter of the Excocomps, on those grounds. (Which btw, if the 24th century had toasters... I think they'd probably look like exocomps, So take that Louvois. Toasters can be sentient too)
Anyhow, that Data deactivates Lore is not an indictment on android sentience or rights. Lore was an ongoing, wholly criminal threat, & nothing short of what was done would've stopped him from being an active one
Let's not forget that Maddox wanted to do more than just disassemble Data. The objectionable part was the core dump of his entire memory & cognitive function, for which he hadn't resolved the hurdle of the "electron resistance across the neural filaments"
He was going to wipe Data. Nobody is planning to do that to Lore