Well, in the comics Slade is against the Titans because one of two reasons:

[hider=Spoilers]1)His son took a mission from the HIVE, that he had turned down, to destroy the Titans. He warned his son not to, but like always he did anyways and ended up getting killed. Feeling solely responsible for his son's death he took up the mission vowing to return the pain caused to him to the Titans.

2)Deathstroke works for an entity bent on world domination. [/hider]