The character 漢 also means "man."
https://www.kanjipedia.jp/kanji/0001100400
See the kunyomi of おとこ and meaning #3 on that page.
Why they used that kanji to represent "man" is another conversation I suppose. But, remember, kanji are themselves from China, so I suppose using a kanji that represents your own people to represent "man" is not that strange for the time they were developed.
Yeah, the second part of this word means man or person.
With the meaning of "idiot" this word's actually been attested in China since the seventh century.
https://en.wiktionary.org/wiki/%E7%97%B4%E6%BC%A2#Etymology_1
The meaning of "Chinese" for the Kanji "漢" comes from it being also the first kanji used for the Han-Dynasty (漢朝) which is a dynasty that became representative for the whole of China or at least for the Han-Chinese people which make up around 90% of the Chinese population.
Why is the word groping; molestor have the "China" kanji in it?
This word and many other words with a negative connotation have the "China" kanji in it. I'm curious as to why that is.