Bit late, but: IIRC the post-credits scene implies that Ultron was somehow really under Thanos' control, via the Infinity Stone Thanos originally gave to Loki (and/or its corruption/influence via Stark via Wanda Maximoff).

I suppose it might be giving the movie too much credit to argue that Ultron was at no point honestly explaining his plans, but instead saying whatever he expected would confuse and/or demoralize his enemies.

The question of liability is sort of alluded to in the latest movie, Civil War; though the short answer seems to be no.

In the end, the only real answer is always "it's all made up and what you see is what you get".