Well the kid wasn’t an agent. He was a student. Who knows what he was thinking. Or even whose side he really was on. It’s rather dramatic that they call him an “informant and agent-provocateur”.
Well, they call him that, because the FBI was using people like at the time. And some of them probably were on the other side. It wouldn’t be the first time such an operation blew up in their faces. If the FBI is going to use a civilian asset, they better be damn sure they’ve vetted and prepared things properly. The planners of these riots may be bad guys, but it doesn’t help if the FBI is, even unintentionally, assisting them.