silver birch nice. Calling someone's little boy sociopathic. Ffs.
Op he sounds like he has an external locus of control. He clearly has theory of mind (understands that little brother can't spell yet) which whilst it doesn't rule out autism as suggested upthread is a point against. He sounds rather self justifying and as if he is used to being encouraged to put his point of view and that this can over ride an adult's view if he dismisses their reasons.
For example, the rock/ lump of mud incident. It could be that he has some issues with shifting cognitively (once he decided to throw he had to throw)
or it could be rather narrow comprehension of language (it's not a rock it's a lump of earth; missing the key fact that the order is not to throw that item;) that would be quite typical in autism
But again I don't think this scenario particularly fits with autism as he made an inferential leap from 'don't throw that rock' to 'its OK it won't smash anything it's just a clump of mud' - he inferred the reason for the ban and defied it on the basis of the reason for the ban, rather than assuming it didn't refer to him as he wasn't holding a rock, if you see what I mean.
Look up locus of control and try to encourage more introspection from him. Label behaviours not the person (sometimes children who externalise so much do it because they have very poor core self belief and it's too painful). Be very clear with boundaries and semantics: if I say don't throw, I mean under no circumstances are you to throw. I don't mean you can over rule that because you don't think it will break anything. If I say stop saying nasty things to your brother, I mean all nasty things. It is not only nasty if the person understands it. If you have hurt someone, you apologise. You don't over rule that because it was an accident.
Do keep an open mind about a possible autism spectrum condition if he frequently misunderstands semantics as illustrated above. It depends if he is genuinely misunderstanding the language or being disingenuous in order to justify his actions, and either is possible.