I think uncle Monty is the perfect example of a Slytherin who is also a decent and caring human being. Here's why (this is more specific to the show because I haven't read the book in a while):
-Ambitious: He is a world renowned herpetologist/recognised scientist. He worked hard for that, and he wanted that title.
-Cunning: That ingenious door that looks like you have to do a million things to open, but really you just need to turn the doorknob. He named the incredibly deadly viper because he wanted to play a trick on the other scientists who were rude to him.
-Protective: Though he may have been a little too self involved to understand that Olaf was after the children and not his studies, he did recognise that he was a threat, and tried to protect the Baudelaires (and his reptiles) from him.
He did of course have some negative traits as well, but everyone does. Overall though I think he should be recognised as a positive Slytherin character.