It looks like you've already answered your question about moral responsibility. I'd like to add that it appears to me, on the basis of this discussion, that Utilitarianism is an attempt to "improve" upon Epicureanism by adding to it. As an Epicurean, to me the additions seek to bring together ideas which do not belong together. I had thought that there would be more in common between the two philosophies but they actually seem quite divergent.
Regarding the greatest good for the greatest number, there's a rather famous short story by Ursula LeGuin that I recommend reading. It's titled " The Ones Who Walk Away From Omelas" and I believe you can find a free copy online. It's an interesting take on the idea.