Well, the question is kinda complicated, but I do my best to answer it.
Severus Snape keeps on making wrong decisions:
- he has an ambition to work for Voldemort - a serial killer and psychopath - when he grows up.
- he hangs out with the wrong people.
- he insults his best friend, who always stood by his side.
- he ends up helping Voldemort and killing heaps of people.
Do you want your children to do anything like that? That's why he's considered a bad rolemodel.
He was never a good role model until the end. He bullied his students, he held a grudge against Harry, simply for bring his father's son and he only helped Harry because he liked his mom.
+He is mean to his students
+He is a Death Eater
+Hurts his students
+Tries to get the most innocent kids expelled
+Best friends with Voldemort
+Is the Half Blood Prince
But he can be a VERY good guy!
