The Michael Clayton star was around four years old when she became resentful at having three brothers and decided to take the life of the youngest child.
But when she approached his cot, she saw the baby chewing his bonnet and removed it from his mouth - and was hailed a lifesaver by her parents.
Swinton admits she buried the incident deep in her memory and only recalled it in 1993 when the murder of two year old James Bulger by ten year olds Jon Venables and Robert Thompson shocked her native Britain.
She tells the Telegraph Magazine, "Years ago, when James Bulger was murdered, every newspaper front page was talking about evil. At that point, having suppressed it for years, I remembered when I was four or five, I tried to kill my own brother. He was newly born and I was disappointed, because he was the third boy. That was enough as far as I was concerned.
"I went into his room to kill him, saw some ribbons from a bonnet going into his mouth, and began to pull them out. And I was discovered saving his life. So I had this strange reputation - my brother's saviour - and no one knew I wanted to kill him. It took the Bulger case for me to remember that I'd seriously wanted to."