Good job.
Just a few suggestions: Work in more music, or just an off button. Both the walking and action music get incredibly repetitive. People like having the option to listen to their own stuff.
Work on the graphics. Gradients are too basic for a flash of this calibre, and the textures can seem really bland at points. I like the style of the junkyard, though. The character design is questionable, too. I like how you're trying to go for a unique, broad shape for humans but some might see it as copying Madness, which everyone's really sick of.
The ending put me off a bit. There's simply an black screen with some text, returning you to the game. This can confuse. I wasn't sure if I was supposed to go somewhere or not after the boss fight. There's a bug with the boss, too: You can fight him by reentering the final area. One more thing: Quickly changing who you're playing as can upset the player. Try to warn the viewer or work a character selection into the beginning of the game.
Under all these minor flaws you have a great flash.