Thanks for all the kind words. If this helps anyone, I'm thankful. The sight of young creatures dead is bad enough, but guilt added to that is a rough combination.
In answer to the question about why I put them out at that age: The weather had gotten really nice out and I felt badly about them being cooped up in the brooder. The were all feathered out by then of course. If it had not been for that gap in the cover on the run, I think they would have done beautifully and it would have been a good choice to have them out there, enjoying the fresh air and pecking and scratching all the fun bugs and dirt. I would make the same choice again to let them out, but I would either locked them up in the coop at night or double checked on that cover.