I think it's natural to be upset and to wonder what went wrong. Like Porkinson with her precious Black brother. And Mrs Malfoy, she mourned him, too. It's only to be expected.

As for who to trust, does Mr Rosier know what made Mr Selwyn decide to turn traitor like that? I mean, if it was something that had a reason, then maybe there are things that would fix whatever was wrong, so Mr Selwyn wouldn't have died for no reason. Or something.