What is the difference between radical initiator and radical inhibitor?
The confusing thing about initiators and inhibitors of radical reactions is that both are often radicals.

But it isn't being a radical that is important: it is what happens when that radical reacts in the reaction conditions. If the radical reacts with another radical to give something that is no longer capable of propagating the radical reaction, it is an inhibitor. If it reacts to give more radicals it is an initiator.

Which compounds can do the job of either depends a lot on the specific reaction.