Unholy by KT Yarrow

He thought he was better off alone. Then a demon found the way to his heart.
After the traumatizing loss of his demon hunting squad, Paladin Luke Morgan swears he’s better off alone. He knows all too well what it’s like to lose the people he cares about, and he won’t go through it again. He keeps his heart shuttered, focuses on the job, and avoids looking at his scarred reflection in the mirror.
When a red-eyed demon named Malachi gives him a tip about a child-killing monster, Luke expects it to be a trap, but he goes anyway, against his commander’s orders. And when he learns it’s not, he’s more confused than ever about Malachi’s motives for helping him.
The truth is, Malachi has been watching Luke for weeks. The moment he saw him, he knew Luke belonged to him. In an effort to get close to him, he approaches with an offer Luke can’t resist: the chance to save innocent lives. He’s determined to chip away at Luke’s carefully crafted armor. They belong together, and he’ll make Luke see it.
But the closer they grow, the more dangerous their love becomes. The Paladin Guild will never allow them to be together. Luke will have to choose, sooner or later. But how can he decide between duty and happiness?