The only thing good about alignment is that it allows people to argue about things like whether Sarevok is chaotic or lawful evil.
Cried every munchkin who just wants to play Skyrim and be a jack of all trades murderhobo.
Sarevok is Chaotic Evil. Textbook definition. Anyone who disagrees either hasn't read the descriptions of the alignments, or failed their comprehension roll.
Saverok was using lawful methods
He was not. He just wasn't being a grug brain and attacking everyone in sight. He had no respect for the law at all.
Lawful characters work within the law. Serevok did not. He subverted the law to his benefit with lies, murder and sabotage, but that isn't the same thing.
It's the difference between a corrupt tax official who uses his position to extort money from people, and a thug who hires a lawyer to help him get away with murder. The latter is a law breaker regardless, the former is not - not technically, at least.
You might be able to make the argument that he's Neutral Evil, but Sarevok's sheer disregard for the lives of others, even including his lover, sets him firmly in the CE camp. He places his own power and pleasure above everything and everyone else.
He's an "evil for evil's sake" murderer, who wants to have as many people die as possible in order to try and become a god. He's a traitor to his government, his organization and even his own father. He even betrayed and murdered his own mentor in retaliation for helping him escape instead of helping him murder the dukes.
There's nowhere near enough of a gray area here to call him Lawful.