The insignificant nobody me understands how you feel, but ULTIMATELY -
1) let peace reign, rather than unnecessary CONFRONTATIONS, which will affect one's own mental state, when there are FAR MORE important personal issues to deal with, than to deal with a seemingly worthless stranger.
2) NO mortals think alike & some may have mental issues.
A) Should the bus have empty seats, just go to one of them. Confrontations are unnecessary as each have their own issues to deal with.
B) Should the bus be filled, & that guy still occupies his space, just a gentle & polite " Sir, may I have that seat, as I am tired after a hard day's work, etc" & for sure he will comply, as Human hearts are made of flesh & NOT of stone, regardless rich or poor, or pai kia ( gangster).
What moves even a presumed pai kai is 理由 ( reason/ logic).
It is a highly regarded term amongst so called gangsters & even the insane. The last thing such rebellious humans would want to hear is 道理 (Rules/Law) which may have religious/societal intonation of long windedness & alienation from what they had felt in their lives...
Education to progress & evolution will never be overnight, but overtime. Have patience, always...