If you keep the scenarios simple and the objects per scenario straight forward then it doesn't really matter if the GM does play the bad guys. Something you might want to consider though concerning GM interventions is, if you a GM+baddie player are starting to gain the upper hand - you can always have a weapon jam rather than suddenly do something radical like give half of the baddies heart attacks. Also - if you are going to play the baddies, make sure you know what they want out of the particular scenario and the personalities of the characters involved, that way you will avoid either losing straight off the bat or winning outright in turn one. Just a thought =)