Gaining and maintaining viewer sympathy for an anti-social protagonist
In a world where the Wealthy [are perceived to] abuse the Poor and Middle class, it is easy to subconsciously maintain sympathy for a 'honest' (passion to teach kids) and 'intelligent' (chem. teacher) person when they perform crime that it characterized in the 'Robin Hood' style.

I've never seen the show and so I'm sure that there are metaphors that make things more hazy than this, but these are broad societal themes in U.S. culture.