ya, i wouldnt be too hard on yourself dave, im sure its happened to almost anyone who has kept boas, almost had it happen to a couple 6 month olds i keep in the same cage, i always stick close by during feeding time and its a good thing i did one time not too long ago as these two babies had each other in a death hold and one would surely have gotten eaten just like your example except that i heard a loud "rustle" in that cage, i turned around (was feeding others at the same time) and saw the two babies like that, got em apart real fast let me tell you and decided to separate at that point 
im also sorry for your loss, cause ive done some stupid s*** and lost a baby or 2
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Brad Hamlin
"...when im a good snake, they sometimes throw me a rat..."
B&P Constrictors