"Attach to a target unit. If attached unit is damaged, destroy that unit."

If I play this card to a unit that already had a damage token on it, would it be destoryed?

or I still need deal damage to that unit to make it destory after I attach "Branded by Khorne" on it?

The forum won't let me copy/paste, but the answer to this can be found in Entropy's Rules Summary. The card only works with damage dealt after it was attached.

Thank you.

After read the rule summary, I think it's a acceptable clarification, even I think the card text is not precisely.