Saw actually didn't follow any logic in my opinion. Like it went too overboard with illogic.
SPOILERS FROM SAW -- DO NOT READ FURTHER:
I only saw the first. But in the first, they never explain why the killer gave the two kidnap victims, tapes in envelopes. Then he disguises himself as a dead body holding a tape recorder. Why not just give them the tapes already in the recorder, ready to play? What if they couldn't reach the tape recorder, then what?
Then they finally find a bullet and the gun that goes with the bullet. Why not have them find the gun and the bullet, already together, ready to go, along with the tape recorder. The killer also says that the blood surrounding him is poison, and wants one guy to poison the other with it. But if the blood was that poisonous then why didn't the killer die, from lying face down in it.
And they never explained that why is it that the killer already had everything ready to go for his previous killings in the flashbacks, but for his latest crime, he likes to keep all the pieces of equipment separate, not knowing if his victims will even find them. He could have done the crime in a much shorter amount of time, but only for his last killing in the movie, does he want to have everything take an illogical amount of time.
Sure maybe it's good to throw logic out the window to make a chase scene happen, every now and then, but Saw had too much throughout, and there wasn't even any pay off to throwing the logic out the window in my opinion, anyways. So I don't know if it's okay for Saw to go that far with it (some critics panned it for that), but most thrillers don't.