riddle - Bathed in violence... Say my name!

In the darkness, I stay hidden, then with the cleansing sweep,
I step out from the shadows, as I watch the whole world weep.

I was torn apart, my body bent, forever segregated.
Broken, I at first give life. Then as a weapon, take it.

Elusive, you won't trap me, nor the hoard kept at my feet,

Sometimes haunted by my twin, though cursed to never meet.

Riding on your gallows, bathed in violence, I start.
Say my name, mere mortal, before I again depart.

Answer

You are a:

RAINBOW

In the darkness, I stay hidden, then with the cleansing sweep,

The "cleansing sweep" refers to the rain. "In the darkness" means that it cannot be seen (unless under some certain conditions).

I step out from the shadows, as I watch the whole world weep.

The rainbow emerges from nowhere. The "world weeping" again refers to the rain.

I was torn apart, my body bent, forever segregated.

"Body bent" refers to the shape of the rainbow.

Broken, I at first give life. Then as a weapon, take it.

"Broken" refers perhaps to the division of the colours of the rainbow. "At first give life" refers to RAIN, "then as a weapon take it" refers to BOW.

Elusive, you won't trap me, nor the hoard kept at my feet,

You cannot contain a rainbow. There is said to be a pot of gold at the end (or "feet") of the rainbow.

Sometimes haunted by my twin, though cursed to never meet.

Sometimes there is a second rainbow on a wider arc.

Riding on your gallows, bathed in violence, I start.

ROYGBIV...

Say my name, mere mortal, before I again depart.

I said already dammit! RAINBOW!

As a personal comment: This is a world class riddle.