Why is dark Rey’s lightsaber unstable?

Why is dark Rey’s lightsaber unstable? The crystal became volatile and dangerous, requiring Kylo Ren to build crossguards into the lightsaber, venting the excess energy to stabilize the blade while also providing additional offensive and defensive capabilities.

The crystal became volatile and dangerous, requiring Kylo Ren to build crossguards into the lightsaber, venting the excess energy to stabilize the blade while also providing additional offensive and defensive capabilities.