These were fit guys, used to pushing themselves physically and mentally. For their exhaustion to prove fatal means the levels the exercise supervisors pushed for were unrealisitic in light of the weather.
The SAS are not super heroes, just highly skilled and dedicated individuals. The mentality that these guys would have adopted would have been a "never quit, never give up" mind set. You dont give up, you dont say no and you dont stop. BUT this has killed them.