expression, ichidan verb
cause one's parents distress; make one's parents suffer
A fixed expression meaning to cause emotional or physical suffering to one's own parents, often through one's actions or life choices.
彼の非行が親を苦しめた。
His misconduct caused his parents distress.
親を苦しめるようなことはしたくない。
I don't want to do anything that would make my parents suffer.
親不孝 is a noun/adjective meaning 'unfilial' or 'lack of filial piety', while 親を苦しめる is a verb phrase focusing on the act of causing distress.
A transparent compound of 親 (parent) + を (object marker) + 苦しめる (to torment, to cause to suffer). The verb 苦しめる is the transitive form of 苦しむ (to suffer).