A good attempt that unfortunately flops
Your biggest mistake in making this, was diverting so very far from the two series you based this upon. While Thing-Thing itself was based off the Madness style (especially for design and animation) you missed this completely, and so the "thing-thing" guy just looked like any other guy. It could have been anyone. I am of the opinion you should have stuck with the distinctive Madness/ Thing-Thing style.
While the still shot art was quite good, as soon as there was any movement the quality dropped sharply, and the weapons, especially the shotgun, looked very poor, especially compared to the still characters. While that is bad enough in and of itself, I've always found the original Madness and Thing-Thing animations and games to have very highly detailed weapons. Unfortunately by marketing this as Madness, you are not delivering what many should be getting.
The music choice was pretty good, and the sound effects were good as well. No need to improve there in my opinion. Sound however was not enough to salvage this.
Now, as for the ending. Viewers will be split on whether it was funny or insulting. I for one got a small chuckle out of it, but at the same time found it very unfortunate that the ending which could have been a decent ending turned into something sloppy and misguided. Ideas that seemed to come out of no where are clumsily handled and don't add to the piece, but rather take away from it.
All in all, what would have been a half-decent flash has been ultimately compared to Madness and Thing-Thing, both of which are quite amazing, and is the downfall of your flash, which fails to emulate the flavour and punch of those two series.