A number of reasons: He has been the Dark Lord's most trusted servant for many years. [Aside from Severus, that is, or so the Dark Lord spent years believing, along with everyone else, but that's another matter entirely.] He successfully kidnapped and posed as Mad-Eye Moody for an entire school year until he was at last intercepted and apprehended. He tortured the Longbottoms to the point of causing them irreparable insanity and took immense pleasure in it. He is a deranged psychopath who cannot in any way be trusted. Just a few reasons but take your pick.
private text;
He has been the Dark Lord's most trusted servant for many years. [Aside from Severus, that is, or so the Dark Lord spent years believing, along with everyone else, but that's another matter entirely.]
He successfully kidnapped and posed as Mad-Eye Moody for an entire school year until he was at last intercepted and apprehended.
He tortured the Longbottoms to the point of causing them irreparable insanity and took immense pleasure in it.
He is a deranged psychopath who cannot in any way be trusted.
Just a few reasons but take your pick.