That's a simple yet very well executed idea. I love the animations when you get to next rage levels and I really enjoy the contrast of the music that fits the game name really well! My biggest complaint would be that for me atleast it was more fun to lose than to win. What I mean is that I had more fun while playing on high anger level than on low. Overall pretty cool game and nice artstyle!
Also one thing it's pretty hard to play in small window, maybe give players some notification on game page to play in full screen or something like that.