Your max hit with dharoks is like 100. Your iron titan's max hit is like 20. If the monkey is at 20 life left, if you let the iron titan finish it, you don't really "waste" much xp. But if you hit it with your axe, you give up 80 damage (320 xp) that you could have had if you whacked a fresh monkey. oh ok, I thought it had something to do with the monkeys bursting into flames if you didn't attack them soon enough. But that loss of xp makes sense. Thanks to everyone who posted.