As anyone who watches The Walking Dead realizes, a good melee weapon goes a long way in sticky situations and can make the difference between survival and ending up the main course of a human buffet line.
While Michonne's famous katana has managed to do the trick thus far, it pales in comparison to wielding a whooshing, luminescent "elegant weapon for a more civilized age."
Sure, a stickler might point out that blood would not be spewing from those walkers since the wounds would have cauterized if she was really using a lightsaber. However, it turns out she was swinging it so fast that she startled the laws of physics, causing a ripple in the universe.
So, that explains...that.