Trading involves money and when it comes, anything can happen such as betrayal and scam even if your most trusted friend is the one you asked to do the trade. You know why? It is their human effort and not yours, the fund is just your but that won't make sense for the earnings are justifiable to supposedly be their own because they have done the job. Now, if you do not want that trouble to come along, better do it on your own self rather than depending it into other people to do the trading for you.
I strongly agree that we must not give the control over our own funds to anyone but there are various API integration I believe such that one can trade with your account but still cannot withdraw or manage that section at all. So, they can only make orders for buy and sell and the worst they can do is get you in loss but cannot scam.