I would say it is unlikely that the females actually killed him. He very well could have died from other health complications that were not visually obvious, and being opportunistic, the girls just started eating away. I have had a few adults die over the years for reasons unknown to me, and I just find their partially eaten body the next time I check the tub. Don't be too hard on yourself (I guess it's too late not to be hard on the girls ) it was likely not anything you could've avoided.

-Evan