Justin seems like the perfect guy when he's first introduced. He's charming, but he enjoys a good dirty joke. It appears as though he was specially tailored to Lindy, and as it turns out, he was. When Lindy walks toward Gareth Fizel's (David Bradley) lair toward the film's climax, it seems like she's about to go head-to-head with the film's big bad in an epic battle to the death. But when she arrives, he's already dead.
It seems anticlimactic, but that's when we get the big reveal that Justin is alive. Not only is he still kicking around, but he masterminded the entire operation. It turns out he works for the government, which wanted to kill Fizel but knew he was too powerful to take out via the CIA. That's where Lindy comes into play.
The government couldn't kill Fizel, but some woman with a revenge streak became the perfect alibi. Justin led Lindy right where he wanted her to go, and she played her part perfectly. One could say the entire operation was all for the greater good, but all Lindy feels is betrayal. Justin may have the button to elicit electrical shocks to Lindy, but it doesn't do much good when Lindy overpowers her treatment, and as expected, kills her former lover.
