Was it ever explained why Slade/Deathstroke was so obsessed with destroying the city and torturing the Teen Titans?

While the show was airing sometime around when I was in middleschool, I had no idea who Deathstroke was, and only started reading up about him after getting into Teen Titans.
From what I gathered, he was never explicitly evil like he was portrayed in TT. He was this mercenary for hire, and had didn't really care about good and evil as long as he was paid well but eventually he moved away from that and just became more of an anti-hero character. Someone who only punished criminals that the big time heroes didn't care about such as druglords or corrupt politicians.