She did not want Larys to kill his own family. She just wanted Larys' father to lose his job so that Otto could return. Larys wanting to get Harrenhal has nothing to with her and was on him and his special "interpretations".

The Larys feet scene is a weirdo show-only addition to her story, though I will address it as if I should consider it canon: The law would be broken if she had intercourse with Larys (like the Rhaenyra - Harwin situation). The stuff Larys does, basically wanking in her presence, would incriminate him and not her. Though it is weird, I will give you that. However, if you had ever read stories about rococo nobles, nothing would creep you out anymore, including Larys' needs.