Warning: This article contains major spoilers for Buffy the Vampire Slayer #9, by Jordie Bellaire, David López, Raúl Angulo and Ed Dukeshire, on sale now.
Among the more interesting changes to canon in BOOM! Studios' Buffy the Vampire Slayer reboot is the journey of each of the core characters. However, none is as radical as the new path traveled by Xander Harris, who was turned into a vampire by Drusilla. However, while his soul remains intact, thanks to Willow Rosenberg, it's now hinted that may not be permanent.
It may only be a matter of time before Xander transforms into the same kind of monsters that he and the other heroes destroy.
XANDER THE VAMPIRE SLAYER
With Buffy and Angel gone in the Hellmouth and Willow preoccupied with her magical study, Xander is left to protect Sunnydale. He's done a decent job of it, utilizing his half-vampire attributes to assist in his hunting. That gives Xander an edge in combat that he lacked in the previous version of Buffy. He's capable of beating, and killing, a vampire that attacks him, with relative ease.
But during the fight, the vampire reveals he knows what Xander is. After he was bitten by Drusilla, Willow was able to halt the transformation by sacrificing part of her soul to keep Xander alive. However, the demon within is still powerful; the process is slowed, not stopped. As Xander kills the vampire, the demon laughs and warns that, sooner or later, he's going to lose control of himself and become the thing he's afraid of.
VAMPIRE RULES
The rules of vampires are very specific in the Buffyverse, and they're largely consistent in the original series: If someone is bitten by a vampire, they may die. However, if they're fed the blood from the vampire that bit them, they may transform into a vampire themselves. When that happens, a human loses his or her soul, and relinquishes control to the demon. The soul is taken into the afterlife, thus stripping the vampire of humanity. While the souls can be brought back through magical means (such as what happened to Angel in both the original and revamped continuities), most of the time it's lost forever.
There haven't been half-vampires in the Buffyverse before this, but it appears that being a half-breed isn't a permanent development, as Xander still has the demon within him. That could end up being the eventual fate of Xander, dooming him to a tragic end. If the series does go in this direction, there's a chance Xander won't make it anywhere close to the finale.