Legal Question in Wills and Trusts in California

Both statutory and case law prohibit a trustee from breaching his fiduciary duties for any reason, and certainly for his own benefit.

It depends on what you are calling a breach- if a bank invests in its own stock, for example, and the stock does well, you would have to get expert testimony to the effect that the same dollars invested in another asset having the same or less degree of risk, would have done significantly better.