Help with the interview question "Why are you leaving (or have left) your job"

I've given notice and am leaving my current company, where I've only worked 7 months, at the end of this month because of my unethical and unsuppportive manager. It has become intolerable and put a serious strain on my health and personal life and I just can't remain there anymore. 

I've already started looking for a new job and have had several interviews and, of course, the hiring managers and recruiters want to know why I'm looking to leave after only 7 months. I give them a mild answer "The responsibilities of the role are significantly different/less than what was discussed during the interview process and don't offer the level of responsibility that I'm capable of." which is true (and much softer than the truth) and goes back to the unethical manager mentioned above. They, of course, want to know more and dig for specifics. I've sometimes given specifics which are negative but I'm thinking this is bad form.  I have also tried just repeating the generic answer until they move on with other questions but I'm thinking this is also bad form.

I'm asking for advice on how to address this in the best way. Do I stick with the generic answer and smile or do I give them the honest details and specifics? Should I say I'm leaving (or left) because of an unethical and unsupportive manger? Or is there another answer I should give?

Thanks in advance.