I do not necassarily agree with Jung on this.
Interestingly his father was a pastor. As a child he used to think he had two personalities.
Maybe he got his idea of a shadow from his own early experiences. The idea that there is another, may be evil side to you. This could well help him survive his two personality existence.
He also developed his ideas with Freud, who blamed everything on early childhood.
This said he has been a major influence on Psychiatry.
From my experience i have had those thoughts in the past. But for me i associate then to a time when i was drinking and doing drugs. I never acted upon them, just thought them. Can't believe i am saying this, i have only ever told my brother. I was only thinking about this today, when i was drinking i had so much anger, this never spilled out externally but i would internalise it and drink on it. However i can honestly say that now i am sober and have a clear head i never think like this and can't imagine why i internalised so much anger and hatred.
Paul