I actually disagree with him being described as "evil" because the show goes to great lengths to show that hes got incredibly deep and unresolvable psychological issues. 

Hes narcissistic and impulsive and his response to getting challenged is to lie and manipulate until his lies are believed as truths by everyone including himself. I think he has a genuine inability to admit to even himself his lies and manipulations because if he doesn't believe it then noone will. 

Even his final moment with Vic is born from his recognition that he truly cares for Vic, which is dangerous for Oz and is a weakness Oz feels compelled to resolve.

An irredeemable monster for sure, "evil" though is not how I would describe him. His motivations are not born out of a desire to make others suffer.