Requiting Hex

If a creature on the battlefield has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

If you can't place -1/-1 counters on any creatures you control (probably because you control no creatures), you can't choose to blight.

If the target creature is an illegal target as Requiting Hex tries to resolve, it won't resolve and none of its effects will happen. You won't gain life even if the additional cost was paid.

All of the -1/-1 counters must be put on a single creature. You can't choose to split them up between multiple creatures.

If a creature that has +1/+1 counters on it receives enough -1/-1 counters to cause it to be destroyed by lethal damage or put into its owner's graveyard for having 0 or less toughness, effects that refer to the counters on that creature when it died will see all of those +1/+1 and -1/-1 counters.

The creature you choose to put -1/-1 counters on doesn't have to have enough toughness to survive the process. For example, if you blight 2, you can choose to put the counters on a 1/1 creature you control.