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It read's the following:
Killing Blow (Su): At 8th level, weapons you use become infused with the power of death. Whenever you confirm a critical hit with a melee or ranged weapon, your attack deals an additional amount of bleed damage equal to half your cleric level. You can use this ability once per day at 8th level, plus one additional time per day for every four levels beyond 8th.
It could be:
a) Always ON but on successful critical hit you choose to activate the bleed effect for additional damage.
b) Not always ON, meaning you need to say or use it before making attack.
Nowhere does it say what kind of action it is, whether it is swift or free since supernatural abilities by default are standard actions to activate.
Domains: ...
In addition, a cleric gains the listed powers from both of her domains, if she is of a high enough level. Unless otherwise noted, using a domain power is a standard action.
I'd say by RAW, it's a standard action.
There simply isn't enough actions in a turn to both attack and perform a standard to trigger this ability.
RAI, I'd say it's a non-action (part of the attack action). If you must, you could rule it some other sort of action (most likely a free, though some might make a case for swift).
In any case: (A).
I'm going to agree with Detect Magic on this one. It was probably intended as a non-action to match with other critical hit modifying abilities.
It is noted that the capacity's effects apply when you successfully confirm a critical hit. Cannot be clearer.
^This^
You can choose to activate the ability as an nonaction. It's not a free/swift/standard/etc, you can just activate it if you score a critical hit.