My baby IRN died, was it killed by it's parents? Please help

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Hi all, I am a new breeder and recently celebrated the lives of three gorgeous yellow IRN babies. They are now roughly 8 weeks old. Just yesterday i took them away from their parents. Two of the babies looked very healthy and big but the third one looked much smaller, and weaker due to the size compared to the other two - so i decided to give the little one back to his mother thinking he needs more nutrients/care/comfort.
The next day the parents were looking stressed - the father in particular was walking around in circles crazily and they both were louder than usual. I was worried if the little one was still being feed. When i checked the breeding box, i found him dead. Quite devastated!!! Parts of his wings seemed plunked and bruised and his tail and bottom was dark (almost like dry blood).
I want to know what happened to him. Did the parents kill him? or was he dying because he was ill? - (strange as to how he died the night after i held him). He didn't look too ill - his wings were beautiful and bright.

It's my first time with baby birds. I would appreciated any answers/opinions/advice. Any reasons why this could of happened? Do you believe I did the wrong thing by giving him back to his mother? (They were only separated for about 30mins). The other two babies have been with me still since separation.

Thank you for taking your time in reading.

RIP. little birdie <3

[MariaFT]

Awww that's so sad. I'm sorry for your loss I've put a baby back after feeding it for a day and night, and the parents took it back with no problems. Maybe it had something wrong with it and they knew?

Sorry have no answers, but sorry again to hear.

[Melika]

I have read in the past from breeding books and confirmed by two breeders that if you leave just one chick, the male might kill it in order to throw the female back into breeding mode.

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Wow. That's insane. The poor little fella .
I did put the two other babies back in for the last 2 weeks and they seem to be doing fine with their parents.
Thank you for replying, very much appreciated. Merry XMAS to all