Desmond seemed to have forgotten that Alex was the one who had rescued them. It sounded more like an instruction instead of a request when Desmond asked Alex for help.
Alex couldn't stand it, since he wasn't obliged to do the Jones family any favor. Therefore, he paid no heed to Desmond's request.
David continued punching Desmond in the face and thought it was a great opportunity to vent his frustration because Alex had no intention of saving the useless wimp.
Ha! I'm no match for Alex, but Desmond's quite an easy target! I'll just beat him up and vent my anger.
Desmond was taken aback because he had underestimated David's combat skills. He couldn't believe that the man was quite a fighter.
I shouldn't have challenged him. I should've let Alex teach this vicious man a lesson on our behalf! Had that been the case, I wouldn't be beaten to a pulp! Urgh! If I don't do something about it, he's going to send me to hell soon!