Credit goes to Moterius for telling me how to fix this:

The "meat" of the kill event is in control.lua.

After the line "for _, entity in ipairs(enemies) do", underneath it add "if entity.valid then". And at around line 42, where you see a couple of "end"s, and yet another one. What you are essentially doing is adding another big loop inside of the for_entity loop.

Keep in mind that I am adding quotes to make this post easier to read. Do not add the quotes to the code.

You'll have to quit and then restart Factorio, not just reload a save game. When the mod works right...instead of getting that crash window you will instead get a little floaty message stating the radius of the kill zone and how many kills it got. Keep in mind that although the initial range of 1 km seems large, its actually only about 31 chunks. The center of the kill zone is the rocket silo, which means if your rocket silo is in the middle of a megabase then you may not get many kills at all. Of course, if you wanted to "rebalance" this, you could always find the code line that says "local radius = 10000 + (level * 100)" and change the 1000 to 10000...