expression, godan verb (-ku)
disobey one's parents; go against one's parents
A set phrase meaning to defy or rebel against one's parents. The kanji 叛 carries a nuance of rebellion or betrayal.
彼は親に叛いて家を出た。
He left home, defying his parents.
親に叛くようなことはしたくない。
I don't want to do anything that would go against my parents.
親不孝 is a noun/adjective meaning 'unfilial' or 'lack of filial piety', describing a state or person, while 親に叛く is a verb phrase focusing on the act of rebellion.
背く is a general verb meaning 'to go against, disobey, betray'. 親に叛く is a specific expression using the kanji 叛 to emphasize rebellion against parents.
A set phrase composed of 親 (parent) + に (particle) + 叛く (to rebel). The kanji 叛 is used specifically for rebellion or defiance, often in the context of betraying a superior or family.