Lore is far worse than Laas. Laas killed that one klingon, but he's the kind of guy who'll leave you alone if you leave him alone. He wants nothing to do with solids.
Lore actively goes out and takes delight in murdering people.
Odo said that the klingon was reaching for his blaster, and Odo's the least likely to lie out of anyone in the series. He didn't need to kill the klingon to defend himself, but unless Odo acted completely out of character and lied for the first time his life, it was self defense.
I believe Odo said that Laas said that the Klingon was going for his weapon. Odo wasn't even there at the time, so how could he make that claim?
There was no reaching for a blaster weapon in that scene. Laas killed the Klingon for no reason, as was implied in the conversation between Worf, Odo, and Sisko in the next scene. You need to rewatch it.