Primal Sin by Ariana Nash

Fallen for love. Risen for revenge.
Severn knows love. He knows how it feels to fight for love, and have it torn from his reach. He knows that when the killing fields are strewn with the dead, and love is all gone, only one thing keeps his demon heart beating:
Vengeance.
His life is now a web of lies. His act, a perfect one. His enemy—the guardian angel Mikhail—sits beside him, turns to him for guidance, and even loves him.
The time has come to make all the angels fall, beginning with the most powerful of them all.
Severn thought he knew love. But he’s about to discover that love takes no prisoners, and as demons and angels clash over London’s battle-weary streets, love will claim the souls of two destined enemies.