I said something like this, to DH! Stephen was a kind, generous, helpful, and wealthy-ish uncle for some decades, and they brought him over for a bigger role, (several years ago,) and have turned him into a mass murdering psychopath. 😱 I wonder why they did that?
Bit like Tim's dad. He was a kindly, friendly, jovial hospital DJ, and they turned him into a psychotic, possessive, controlling arsehole who nearly ruined Yasmeen's life.
Same with Richard Hillman. Was a nice, friendly, happy-go-lucky family man (and was Alma's cousin IIRC,) and he turned out to be a murdering psychopath too.
Only difference is that Tim's dad and Richard were in it less than 5 years, and Stephen is a character that goes back several decades - (first mentioned in the 1980s I think???)