Lust: The Bad Earl by Christi Caldwell

Ruthless, emotionless, and deadly, Severin Cadogan, the Earl of Killburn, was England’s greatest secret weapon. One misstep put an end to his respectable career as sanctioned killer and informant. Fortunately, there’s plenty of evil to go around. When he’s approached by a potential client with a proposition involving the gentleman’s sister—it’s the last assignment Cadogan wants. Alas, he’s been made an offer he can’t refuse…and one he quickly finds himself enjoying for all the most depraved reasons.
Lady Raina Goodheart, the Ton’s Diamond, comes from a line of ancestors known for their wickedness. As for Raina, she yearns for true love with a respectable gentleman…but what if she really is no different than her notorious family? That fear is further fed when Raina meets him—Severin Cadogan. He is darkly dangerous, soulless, coarsely forthright, and she is instantly spellbound.

The more time Raina spends with Severin, the deeper into debauchery she falls with the dark lord. Even more perilous, it isn’t long before Raina’s falling for Severin, a man with a heart of steel.