Since the trigger in this case is off a target's death, the answer is before the death occurs.
The Hit line takes effect all at once. Resolve the attack, then deal with the consequences. (You could actually infer from the color text in this case that the damage is caused by the teleport, but either way both damage and teleport happen together before anything else.)
If the successful attack had triggered an immediate interrupt, that would have happened before the teleport and before the damage.
As noted in the comments by @Pat, effects that are triggered should be understood as immediate reactions unless explicitly stated otherwise. (Or unless that prevents the effect from working: "reroll on a miss" will always interrupt the miss.)
From the Rules Compendium pg 197:
If an effect has a trigger but is
neither an immediate action nor an
opportunity action, assume that it
behaves like an immediate reaction,
waiting for its trigger to completely
resolve. However, ignore this guideline
when the effect has to interrupt its trigger to function.