Actually, wal-mart loves when employees take the initiative, and you've just given an example why - they need good managers, badly. If you've got a decent head in your shoulders, it shouldn't take you more than a couple weeks to get your own department, and a couple months to get a management position.
involves mastering how to deal with the emotions of your coworkers in a productive way though - when to push, when to give, when to do what your told, when to do what's important, who's reliable, who's not, and when and who to pass things off to.
And most importantly - don't leave messes. If you don't have someone you can absolutely trust to take care of something, assume it won't get done, take charge, and take responsibility. I'll note that your manager seems to have screwed this bit up royally, but it was your initiative, so you did too.
Of the many members of my family who've played the walmart game (and ended up with the owners practically begging them to join management) I'm the only one who didn't pull it off. And that's because I a) worked back room for more than the managers made and b) when they eventually shifted me to the front and cut my pay in half, I spent six months sleeping through my shift in the bathroom.
I was not an ideal employee, heheh. The rest of my family was, though. (All at different stores, no favoritism going on)