Stop that. If there were no signs then you would have to be clairvoyant or a vet to know. I could say that I should have known my boy had a stress fracture (or whatever was wrong with him for his leg to snap to easily), but I didn't and I don't want you beating yourself up over something that you can't change, and you couldn't change if you were to relive the last week.
You didn't fail him. These things definitely happen (and have been happening). There are worse ways for a horse to go, and you know it, babe.
Take the big stick and put it away, just grieve and be sad and send him big love messages.