I think the most likely route to destructive AI is a corporation with an AI CEO - possibly one that takes over from a human CEO in a boardroom coup.
Corporations already have legal personhood and act in their own interests. It's going to be much easier to automate and formalise business decision making than to develop a true general intelligence with a full spectrum of human characteristics.
This may sound like science fiction, but as competence increases shareholders - who typically are only passingly interested in moral issues - are likely to demand the increased returns an AI CEO can bring.