I Think A Lot Of People Haven't Actually Read Flatland So You May Not Realize A. What Bill's Eye Mutation

I think a lot of people haven't actually read Flatland so you may not realize A. What Bill's eye mutation means and B. What precisely Bill did to destroy his homeworld.

Bill's home isn't completely the same as Edwin Abbott's Flatland: A Romance of Many Dimensions (notably, women and men can be both polygons #feminism) but we can assume most of the mechanics are the same. The basic premise is that the world is 2D. Everyone perceives the world in a 1D way, along the plane. There's a part of Journal 3 that describes this pretty well.

Bill can see up to the stars because his eye is on the flat surface instead of on the side like everyone else, like this:

(He's also slightly 3D, as we can see in the show.)

But there's one more important Flatland detail. The denizens of Flatland (and therefore likely Euclydia) do still have organs "inside" their bodies. Since there is no depth, they're just on the inner radius of their bodies. The 2007 Ehlinger movie adaptation shows that:

If Bill wanted to "give his world a new perspective" and "show everyone what they were missing," he wanted to get everyone to look upward.

Meaning he probably tilted the entire world.

Meaning everyone not only slid off of the plane, but all of their organs spilled out and everyone died.

...Hence "so much blood."

(The only issue with this is that it doesn't account for the number of times Euclydia is referred to have been burned ("saw his own dimension burn / misses home and can't return", he only has ashes leftover), but I'm sure the act of turning an entire dimension upwards expends a lot of energy.)