Cheating as the gmDepending on how you look at it, the gm cannot really cheat in a roleplaying game. Its their world, after all. However, its clear there are lines gms should not cross. Generally speaking it shouldn't be the gm's role to tell players how they act, unless said players are clearly using knowledge their character simply would have. Also, while some dice fudging might be allowed, the rules he's using should be the same as applied to the players. Or, if not, the rules are consistent (asymmetric rules between players and gms are not unheard of, but these do need to be clearly defined.)
What kind of dice fudging should be permitted? Ideally it should be avoided as much as possible, but I think sometimes should be allowed. While preventing your players from ever dying is not fun, its quite easy to accidentally throw an overwhelming encounter at your players by accident. In those circumstances it might be worth ignoring a couple of things. I've done this once or twice, but it is best to avoid, becuase players can't have accomplishments if you're holding their hands. Its also best to avoid fudging for anything other than the players favour, no matter what happens. If, for example, your swarm of unstoppable nurglings get taken down in two rounds of combat, then you just have to live with that, however implausible (between them the npcs rolled 4 ulrics furies....).