They'd be Lord "Name" of the House of "Dynasty Name". They'd have less legal influence than a lower rank with an actual title but so long as the title holder backed them they'd have the defacto influence of the House standing behind them.
It's one of those areas where the formal legality and...
They couldn't not kill her. She actively harmed a genuine Saintess who in the end saved probably most of the population of their kingdom. Even if you ignore the secular crimes that veer dangerously close to outright high treason, ignoring that type of religious crime or "crime" would mean that...
You say that like there aren't an immense amount of people IRL who suddenly do a complete 180 on a topic simply because the end circumstance has changed a little.
If it's a stupid plot development, it's a stupid plot development that's happening in the thousands at minimum at any given moment irl.
There's a lot more than 2 ways. Those are the most likely options but there's also "Some past event traumatised him" "He fell in with a negative crowd and attempts to pull him out just lead him to dig in deeper" "His elder brother holds views that are completely the opposite and jealously lead...