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.
“At times the world may seem an unfriendly and sinister place, but believe that there is much more good in it than bad. All you have to do is look hard enough. And what might seem to be a series of unfortunate events may in fact be the first steps of a journey.” ― Lemony Snicket