Don't Think of Lissandra as an 'Anti-Hero'...
Think of her more as a 'lesser of two evils' compared to that of the Watchers. She was obviously an ambitious, somewhat power hungry woman for making a deal with the Watchers to grant herself magical abilities and immortality, but she obviously knew the true threat of the Void (something that Malzahar seemingly embraces or outright is unaware of).
She is still the same Ice Witch who manipulates Freljordian history for her own benefit from within the shadows, only she does it for herself now rather than for the Watchers. The fact that she fears the Watchers so much and is so active in keeping them imprisoned only emphasizes how big of a threat they are, maybe even as the true 'big-bad' of League's universe.
, then her current bio really needs a rewrite to emphasize that she's still a manipulative, power-hungry villain. There's way too much emphasis on the Watchers and her not working with them.