Pestilence - The Four Horsemen #1 by Laura Thalassa

When Pestilence comes for Sara Burn's town, one thing is certain: everyone she knows and loves is marked for death. Unless, of course, the angelic-looking horseman is stopped, which is exactly what Sara has in mind when she shoots the unholy beast off his steed.

Too bad no one told her Pestilence can't be killed.

Now the horseman, very much alive and very pissed off, has taken her prisoner, and he's eager to make her suffer. Only, the longer she's with him, the more uncertain she is about his true feelings towards her... and hers towards him.

And now, well, Sara might still be able to save the world, but in order to do so, she'll have to sacrifice her heart in the process.