could I let warlock take demage to die than destroy it to use spicial ability(I have drawn card can use spicial ability)

I'm not sure I really understand the question. If the Warlock is alive for Initiative Round 3, you will draw cards for it; since the Warlocks power isn't dependent upon it being alive when resolved, if the attacker ends up killing it, the Warlock power will still trigger.

I think it was happened in the same time. warlock take demage and trigger his ability

darkhells said:

Yes, if the Warlock is killed, his cards are still played.

However, the only way a Warlock could be killed before using his special ability is if another unit used their special ability to kill him; Orbs are resolved first, then routs, then damage.

Although with the Warlock, it doesn't really matter that much.