Why was Chancellorsville both a victory and a defeat for the Confederates?

It was a victory since the Confederates literally won the battle, but it was a loss to them since one of their really good generals Stonewall Jackson was mortally wounded by Friendly fire and died about a week later.

Chancellorsville is known as Lee's "perfect battle" because his risky decision to divide his army in the presence of a much larger enemy force resulted in a significant Confederate victory.