My take, the hill I will die on is that regulus was not forced to take the dark mark or become a death eater.
I think it’s integral to his character and everything that makes his story that regulus chose to be a death eater. Was there pressure in his environment and his upbringing? Maybe! Did he grow up around people that would have influenced his choices? Yes! But regulus still made that choice. I don’t think his parents or anyone else forced him.
He chose to be a death eater, be believed in the blood purity rhetoric. He then chose to defect and try to take down Voldemort and that makes his story so much more impactful because he went against everything he’d been taught and everything he believed to get the horcrux.
Regulus was a bad person who did bad things, and he then died to undo them and try to put those choices right. Don’t glaze over or water down his character, don’t twist him into just the Poor Good Guy. it takes away from everything that makes him so distinct and interesting.