If you think Diem was a democratic ruler, then I suggest you revisit your Vietnam history. Part of the reason we got involved in Southeast Asia, and wanted to maintain the partition, was that we knew Ho Chi Minh would have won in a real election.
But that's all water under the bridge. As for your point about the mustard gas, if Gaddafi or a third party makes a move on the mustard gas facility, we'll know about it and we have the option of taking it out with an air strike. Still no real need to get ground forces involved.