Character Analysis
For every brilliant leader, there's a second-in-command who doesn't quite like being lower on the totem pole. And for every less-than-brilliant leader, there's a second-in-command idly toying with murdering him in his sleep.
Enter Wally, the guy who thinks he could be doing a much better job than the guy in the big chair—and who never hesitates to let everyone know it. We never see what they were like before running off with the map, but it's very clear that Wally doesn't much like the job that Randall is doing:
WALLY: Horseflesh wouldn't have got us into this.
RANDALL: Horseflesh is dead.
WALLY: Then give me the map. I'm taking over!
RANDALL: Get off me! Get—
Wally is itching for a change in leadership, and, of course, he thinks he's the guy for the job. That doesn't make him any more qualified than Randall, in all likelihood, but, of course, he thinks differently. You can see his self-regard when they break out of the cage in the Fortress of Ultimate Darkness. He's all about performing dangerous tricks after escaping the cage instead of doing the wise thing and just getting to safety.
Dude has got a hot temper, too, and he's not the kind of guy who will just sit around and fume. When he gets angry, he acts, and while his actions are usually pretty futile, you can't argue that his heart isn't in them. His anger at Randall's leadership, for example, causes him to throw a skull at the invisible barrier, revealing the Fortress of Ultimate Darkness:
RANDALL: We've made it, lads! We're here!
WALLY: You're mad, Randall! Your brain's gone. Do you know that?
RANDALL: I promise you, this is it! We are here!
WALLY: We're nowhere, Randall!
That puts Wally in the same boat as the guy he thinks he's so much better than—but, like Randall, he's also loyal to his friends and appreciative of what Kevin brings to the table. He never says so directly, but he also never turns his back on them.