Well, just like an above poster pointed out, he may have been trying to get them to cut both the blue and red wire (but I don't recall right now if he was really trying to push this option).

I also think that the treadmill challenge was a situation in which it would be difficult to sabotage without being obvious. No one would believe that he needed to stop running, because he's such a strong runner. I'm still convinced that Craig and Nicole were deliberately answering the questions wrong, or taking too much time on easy ones in order to see if Mark would step in and answer. If he didn't step in then it was confirmation in their eyes that he was the Mole.