I've come to believe that a deep childhood wound (parental divorce, neglect, abuse, or being adopted, etc.) predisposes one to become a narcissist when combined with the message that you are special, different from others, etc.

Steve Jobs, for example. He wasn't as malevolent as Donald Trump, but both have both factors: the childhood trauma + the message that they were special. I have several acquaintances who are narcissists and have both factors as well.

I think that the trauma without the message that you are special and different predisposes someone to become a hyper-achiever, where ambition is a channel for the pain. See: Elon Musk, Tim Ferris, etc.