Pilot error caused the crash of SpaceshipTwo.
The feathering system, which rotates the tail to slow the vehicle down when descending, was inadvertently activated during the boost phase. The airframe was not designed to handle that kind of stress, so naturally it came apart.
One of the worst aspects of the advancement of technology is the fact that sometimes people die in the process. Okay, when they were first experimenting with attaching wheels to steam engines to turn them into locomotives, they didn't immediately install modern Janney couplers; they used all sorts of coupling methods which were dangerous, especially to the guys who had to stand between cars being coupled, such as link and pin.
The federal railroad regulations in particular are written in blood, because whenever there's a fatality the rules surrounding the incident are reviewed and modified as needed in an attempt to prevent further death. If you've ever wondered why trains have to blast their horns in that particular long-long-short-long pattern when crossing a road, for example, it's because it was a pattern that was not used elsewhere for notifying people of what the train was doing, and it's evocative of the syllabic rythm of "cross-ing at grade"...and it's distinctive enough that you can play the pattern on any instrument, and anyone who's ever seen trains in operation will know what it is.
It's much the same for aviation. The DeHavilland Comet was the first commercial jet airliner, and there were two crashes linked to metal fatigue, mainly around the square windows. These crashes were why pressurized airplanes have small, nearly round windows.
We're learning how to do these things and that makes it dangerous. Someday it will be a lot less dangerous, but that won't happen without accidents and mishaps that teach us how to do it safely, and some of those accidents will be fatal ones. It's sad, and we do everything we possibly can to prevent it, but ultimately, this is how it works.