[noir]

Indeed underestimating one's opponents is a fatal flaw; however factors such as big bets to reach showdown and position are trumped by the possibility of winning combined with the fear of losing should his hand be best or become best. This is his psychological weakness.

Ours can be projecting our own understandings of the game and particular situations onto others unnecessarily for the purposes of retaining hope and/or executing what we believe to be the optimal play. We take solace in the fact that we played the hand well - because that is what we tell ourselves even though we did not.

With regard to the hand, I'd fire. Their range of hands and the fact that they have in no way defined them as pairs higher than eights justify it. Previous action and feel could swing it to a prudent check though.