I just started watching this show Luther, and in the opening scene the detective has a chance to save the life of a serial killer.

If he saves his life, then the serial killer will go to trial, may be convicted, and will spend the rest of his life behind bars (no capital punishment in Britain). There's always a chance though that the serial killer won't be convicted, or will escape or be paroled... and may kill again.

If he doesn't save his life, then the serial killer dies and assuming he was guilty, his victims get instant justice. Of course if he wasn't guilty, then the detective just served as judge, jury and executioner.

Ok so here's the situation from the opening scene of the show. The detective is convinced that someone is a serial killer, confront him and they then reveal information that only the killer (or someone close to the killer) could know. Then the person falls through an old wooden plank on a bridge and is hanging on for his life.

Assuming that no one would know either way and that you were the detective, would you save the life of the other person?