Created by Marv Wolfman and George Pérez, Deathstroke is the Boba Fett of the DC Universe — he’s disproportionately popular because he looks super cool. The split black-orange mask, with an eyepatch and sword? How could any preteen boy not love this guy? Even his real name — Slade — is badass.
Deathstroke began not as a Batman villain but as a Teen Titans villain. His son, Grant Wilson/Ravager, accepted a contract to kill the Titans and died while carrying it out. Deathstroke takes up the contract in Grant’s honor. This leads to “The Judas Contract,” where it’s revealed the Titan Terra is really Deathstroke’s inside woman on the team. If you watched the 2003 “Teen Titans” cartoon, you might also remember that Deathstroke was the main villain of that show, albeit just called Slade (with a chilling voice provided by Ron Perlman).
Deathstroke’s enmity with the Titans gives him a tangential connection to Batman — after all, the team is led by Nightwing/Dick Grayson, Batman’s old protégé. But it’s just that — tangential. Still, Batman is DC’s most popular character and has eclipsed Superman as the center of the universe, especially in films and TV. Pairing DC’s most popular hero up with one of its most popular villains makes cynical business sense, like putting two prized fighters into the same ring.