Dark Vows by M’Renee Allen

Though she was sent to kill him, she ended up becoming his wife.
Usually, when someone tries to kill Andrei Kuzmin, he lets them live long enough to regret the day they were born. Yet, when a beautiful assassin crashes his wedding, he sees it as a good omen. He needs a wife more than he needs revenge. His contract bride has run off, ruining all of his carefully laid plans. Now, he’s left with a hitwoman in his bedroom. His little assassin has two choices, a wedding or a funeral.
She chooses a wedding and changes both of their lives forever.