When concentrated, negative energy can be destructive. Some of the ways of delivering potentially lethal does are energy weapons like black siege crossbows, Nighthowlers, and negative energy grenades.
One of the most effective ways to channel negative energy is by using a material called bonebrack. Undead using weapons made of this stuff can channel the effects of their touch attacks through the weapon; such that a ghoul can cause paralysis through a bonebrack weapons as if they touched the victim.
The largest sources of negative energy are on Cubes of Negativity, better known as Death Balls.