I love the idea for the limitation! However, I never really made use of sparing enemies, because the game was too easy. I think you should add more levels and make them more difficult, so that the player actually has a good reason to not kill enemies.

I also loved the charming graphics and the Super Mario Bros. inspired level design. Good job! 

I liked the litte guy and had a fun time completing the game. 

I had SuperMario vibes while playing the game, but it is not a bad thing, the game is cool, nice entry :)