Cut Glass: Jewels - Ruby. Cut Glass, #2

Forget everything you think you know about mass murderers and crimes of passion, because now you're going to learn the truth. People don't decide one day to go out and kill as many people as they can, or wake up one morning, grab the nearest shotgun or axe, and proceed to hack apart their loved ones. They do it because they're infected with a jewel, a creature of intense beauty and danger. Once the jewel takes over a host's body, that person dies, leaving the jewel in total control to thrive on fear and horror.
Cutler and David are reunited, and this time they will fight anyone who tries to separate them again. But when someone steals shards of defeated jewels, word is that an underground faction is trying to re-grow the deadly entities from those pieces. Cutler and David are assigned to help recover those shards before that can be accomplished. Because if they fail, the number of jewels will increase to where the delicate balance between lapies and demons will be compromised.
After which the wholesale slaughter of mankind will be imminent.