The creepy Volker guy probably killed his assistant as a warning to Lisbon, or to mess with her, rather than anything to do with the assistant herself.
I love how well thought out this show is - they've clearly planned ahead. I'm about 65% sure that the guy from Homeland security isn't Red John, but that he is somehow connected is pretty clear... But I haven't been so engrossed in a crime drama series in a long time. I even prefer The Mentalist to Castle, which I loved before I found The Mentalist. And I just found out recently that Patrick Jane is played by an Aussie - way to go, us! :p