- Benjen must be in play to trigger his interrupt (like all non-events, unless stated otherwise on the card). So he cannot trigger from hand or from deck if hit by Heads on Spikes or Ser Gregor Clegane. Furthermore, his interrupt is to being killed, not to 'being placed in the dead pile'.
If a Night's Watch player is at 13 power, including 1 power on Benjen (through Longclaw, Consolidation of Power, etc.) and Benjen dies, does he reach 15 power and win the game? Yes. RRG pg 10. "Always resolve interrupts to a triggering condition before resolving the consequences of the triggering condition itself." In other words, Benjen's ability puts 2 power on your faction before the "death" actually completes and causes the usual leave-play loss of all power on Benjen himself.
- Benjen must be KILLED to use his ability, not DISCARDED or SACRIFICED. These are specific game terms that refer to specific occurances.