In my opinion, he is going after the people that are confusing the liberty movement. Clinton is a known commodity. Cruz pays lip service to the liberty movement, but he isn't fooling many of us.
Trump and Sanders, on the other hand, are tapping into the same contrarian streak that lives in this movement. It's easy to rail against the system, but the alternatives that these guys are presenting are in the complete opposite direction of liberty. Ron Paul is trying, and has spent a lifetime trying, to steer people's anger towards the real culprit - governmental control. Both Sanders and Trump represent more governmental control and less liberty. Clinton and Cruz do as well, but you already know that.