I hoped you were going to ask that question! Instead of using bullet, you can use the same 8direction behavior that you use on the player object. You just need to turn off the default controls and manage it yourself using collisions with walls and other things.
For example, in the example you show, when the monster hits a wall, add 90 degrees to the monster's angle to help turn him around.
Here's a quick example I threw together. There is much more you can do. In addition to managing the collision with the walls, this one has some controller objects that turn the monster left, right, up, down, pause, slow down and speed up. During the deployed game, turn the visiblility for the controllers layer to invisible and no one will know what is making the monsters behave the way they are.
This is based on some of the great platformer patrolling examples out there, like JohnnySix's patrolling example in the arcade. Of course, his is very polished, while this one was thrown together in under an hour.