If you forced him to atone and become of good alignment, he'd quickly slip back into evil once you stopped watching him. Alignment is just a reflection of your actions, not the cause of them.
And V's actions here, while borderline evil/chaotic, was simply the right thing to do. The way to see it, is that he didn't really surrender, he just took advantage of Elan's morals to survive another day and backstab him later. Not unlike a villain who drops his weapon only to pick it back up when the hero looks away.