It looks like Entity.tick() can remove itself from the entities list? If that's the case, then you would need to keep a separate list of entities that were deleted and delete them after running through the tick() loop. Alternately, if an entity can only ever delete itself, you could do a reverse 'for' loop (instead of a foreach) which would keep the removed item from affecting the tick() loop.

Only problem I can see with that is the fact that it would seem EVERYTHING needs to run through the tick() loop to actually run. I'll see what I can engineer, and maybe I'm not understanding it quite right, but let's see.

I'm not figuring it out. Anyone else have an idea on how to fix this?

This question seems to have morphed. Can you restate the issue you're working against now?

Sorry, your right...
I'm working against a ConcurrentModificationError

you have probably a nested remove() in a iteration over the list itself.

you can use a list or set that works with concurrent access or you have to que all read and write properly in you game loop.