Still, as far as the yautja go I don't really think they underestimate as much as they don't trust. As Hunters their lives are constantly on the line, they can't afford to underestimate. Trusting something to not attack you is a poor decision, and questions one's own intelligence. The Hunters don't trust anything so they kill it. Being as they are not even human, how can we compare them to human standards of mercy and ruthlessness? They have an Honor Code, human also have one to an extent, but as it's clearly evident both condratict on many areas. So no, I don't think yautja underestimate much, just make bad decisions.
Hmm...now that I think of it, isn't it similar?
~ No one is taller than the last man standing ~
~ Males conquer, females rule ~