These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
Paying Life Points is mandatory, not optional.
Previously Official Rulings
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official, but may or may not necessarily hold true nowadays.
If you don't have enough Life Points to pay for an Archfiend, the Archfiend is destroyed and you don't pay the cost.
You roll a die for Archfiends when your opponent resolves an effect that targets the Archfiend, not when it is activated. If the activation or effect of a card that targets your Archfiend is negated by some other card, then you do not roll a die for the Archfiend.