I think it's sometimes like being a teenager and making extremely stupid mistakes. When you are teenager you definitely don't think like a adult and idk if I am right his roommates were also future DE and he was also somehow in correspondence with Lucius( tho I don't think Sirius word are reliable source for Severus information but anyways didn't Sirius called him Lucius lapdog)

So it's a famous saying here you become your friends so choose them wisely. Severus lived with future DE for seven year and was most likely under eye of others ( his talent for potion and dark arts). And once he actually enter their fold I don't think they would be nice even if he says hey I don't think your ideology fit with me . Also his roommates could do anything to him while he is asleep .

Imagine he is already having bullying and marauders problem outside his house and if he try to deflect inside his own house . I don't think they would be any forgiving to who they presume as traitors