I love it! Definitely an interesting little game. I killed everything in sight in the first run and the ending made me think that there must be another way to do it. The game felt tedious in the first run, but when your aim is to kill the green moster without killing anyone, it becomes more interesting and not as tedious.
I didn't like the 'why' text either, since it seems out of place and too much 'in your face' about the theme. I think the way each character in the game response to the monster already tell a lot, especially with the random speech and how they crawl when they're hurt and almost die. Adding the floating red text keep asking 'why?' is just too much, imo. Although I admit that it worked as a sign telling you which ending you're going to get.
I'm not sure about adding enemies that attack you. It might be more interesting, but it might be tedious as well. Trying to get rid of the slime without hurting anyone is already a tough task as it is!
Anyway, great game and great theme. The good ending is a bit of a let down since it ends abruptly, but I enjoyed the experience.