Continuity mistake: The amount of blood on Alison's padlock in the first of the group traps changes instantly. (00:22:00)
Continuity mistake: In the beginning of the movie, Strahm reaches out into the hall to grab the tape player. The string it is hanging from comes with it. Strahm exits the room, and we get a reverse angle and the string that held the tape player is hanging from the ceiling, but empty. The string should not be there to begin with.
Continuity mistake: When Ashley is decapitated, a pool of blood appears under her severed head. Later when we see the head from the second room, the pool of blood is not present. (00:26:50 - 00:33:30)
Continuity mistake: When entering the second room, the position of the door, before it gets closed, changes depending on the camera angle. (00:33:15)
Answer: The games of Jigsaw and his followers were always intended to be extremely difficult, but with a small chance for survival based on the actions of the player. Hoffman probably wouldn't have cared either day if anyone else survived, but likely anticipated that most of the others would die. (Strahm's line about everyone being supposed to die with Hoffman being the soul survivor was more conjecture than anything.) In terms of killing Jeff- both Jeff and Strahm were emotional hot-heads, so in all likelihood one or both was going to die if they encountered each other. Strahm's water cube was intended to be an execution tool for breaking the rules. That's why Hoffman seemed so shocked that he miraculously survived.