Stupidity: Cole fell into the lake, and he said he couldn't swim. Instead of making any move to avoid being mangled by the boat motor Becky was aiming toward him, he just stayed in the same spot and was killed. Even if one cannot swim, it would be possible to hold one's breath under water for a while to escape the motor's range. It also seemed like he was close enough to the dock to simply lunge forward and grab onto it to stay afloat. Cole was close to the lake's edge - the water shouldn't be deep.
Stupidity: Becky possessed the mysterious key that the four neo-Nazi prison escapees were at the lake house to retrieve. Becky didn't know what the key was for, so did not have a vested interest in it. By first denying that she knew where the key was and later refusing to give it to Dominick, her father was killed. Had Becky simply handed over the key when Dominick asked for it, it is possible no-one would have been hurt/ killed. Becky keeping the key with no importance to her served to further the plot.
