Can you sacrifice a creature to an activated ability in response to a spell that targets it?

When an opponent casts a spell that targets your creature, that spell goes on the stack. Before it resolves, both players receive priority and may take actions — including activating abilities. If a sacrifice outlet's activated ability requires you to sacrifice the targeted creature as a cost, you can activate it right then, removing the creature from the battlefield immediately as part of paying the cost (CR 602.2a).

Once the creature has been sacrificed, it no longer exists on the battlefield. When the targeting spell later tries to resolve, the game checks whether all its targets are still legal (CR 608.2b). A creature that has been sacrificed is no longer a valid target, so the spell is countered by the rules and does nothing — it simply fails to resolve.

This is commonly called 'sacrificing in response' or letting a spell 'fizzle.' It is an extremely common play pattern because sacrifice abilities are activated abilities, meaning they can be used any time you have priority and the ability itself doesn't have timing restrictions (CR 602.2, 116.1).

Concrete example: Your opponent casts Murder targeting your Reassembling Skeleton. In response, you activate Viscera Seer's ability ('{T}, Sacrifice a creature: Scry 1') and sacrifice the Skeleton as the cost. When Murder tries to resolve, its only target is gone — it is countered by the rules and does nothing. You also get to scry 1.

Note that if a spell has multiple targets and at least one is still legal, the spell resolves but only affects the remaining legal targets (CR 608.2b). You only fully 'fizzle' the spell if all of its targets become illegal before it resolves.