Charlie was an interesting kind of monster. he was a 17 year old without any human guidance whatsoever. He was obviously quite curious about so many things ye he knew nothing about relating to others. in some respects he was quite similar to some teenagers who can be incredibly self-absorbed, but those kids don't have the power to make their annoyances vanish or melt into a puddle. Couple that with a lack of impulse control that quite a few teenagers can experience.
The episode was decent except for the ending. Why couldn't those beings just take his power away and give him back to Kirk as a regular human being? It's clear Charlie X didn't want to leave. All he needed was someone to remove his capacity to hurt people, and that way Kirk could start dealing with him on equal footing.
I'd give it a 7, but the ending reduces that to a 2.