Arjan, I think it's a lot more grey than that. I don't think it is so black and white as if you use too much you are a Warlock. The Council has definitions that don't necessarily match up with the universe's. Jim said something about that when people asked him about the Laws of Magic.
The council's laws are rough approximations which is inevitable because though black and white exist there is also a world of grays. But being a warlock does not necessarily mean you are a compulsive idiot solving all your problems with black magic without any insight in your situation.
It depends on your definition of warlock. The council would say that everyone who broke the laws is a warlock. That keeps things simple. All other definitions have grey areas build into them.
If you keep it more vague the term warlock tells you what you can expect from someone. It tells you something about his nature that can be changed using free will but so few people do.
The use of magic, not just law breaking magic, is strongly based on your nature and free willed choices to use your magic have a strong influence on your nature. The laws of magic shape the wizards believing in it. Lucio did not use magic to kill LaFortier because she could not even in the state she was in.
Black magic is said to be addictive so that has to be taken into account.
In that sense Harry is still a warlock, his barriers from using black magic are too low, he has to be carefull. But he tries not to use it and he is quite successful at that. In that sense he is not. If you trust him that is.
Grevaine was unhinged but he knew what he was doing and Cowl was actually avoiding breaking the laws if he did not think it necessary. But cowl is an outsider agent and that influence can disturb the picture. The same for Peabody.
That terminal state of crazyness the korean boy was in is not the only type of warlock and just the least dangerous one.