I'm so sorry--I posted not long ago about doing exactly the same thing, and I felt exactly the same way you do now. We blamed raccoons at first, since early on we'd lost three babies to raccoons, but the night I left the door to the chicken yard open (and 23 out of 28 chickens were killed), we're pretty sure now a fox, or family of foxes, were the culprits. We set up a motion-sensor gamecam, and captured the fox making multiple appearances on following nights. Thank goodness for electric fence.
The aftermath of our massacre was 7 dead birds lying on the grass, without a mark on them, and 15 birds *vanished*. It was a nightmare.
It's a hard lesson to learn, but I bet I won't be forgetting to lock those birds up again for a long time to come.