"...is he guilty because he thought it?" I don't think so.

He was convicted of conspiracy, defined in the US as "an agreement of two or more people to commit a crime, or to accomplish a legal end through illegal actions. Conspiracy law usually does not require proof of specific intent by the defendants to injure any specific person to establish an illegal agreement. Instead, usually the law only requires the conspirators have agreed to engage in a certain illegal act. This is sometimes described as a "general intent" to violate the law." [Forgive me, I snagged that from wikipedia]

So, he was in no way convicted of thinking, or fantasizing about bad things, he was convicted of conspiring to do those bad things.
The "because he thought it" sounds like desperate weasle-ese on the part of a doomed defense.