sacrifice a creature about to die

[later]

can you sacrifice a creature who is about to die?

[Arix]

Depends what you mean by "about to die".

Targeted by a spell like Doom Blade? Yes, you can respond to such spells as normal.
Assigned lethal damage? No, state-based actions will kill the creature before you get priority.
In response to something that would deal it lethal damage? Yes.
Being sacrificed for another effect? No, you can't satisfy two effects with one sacrifice.

[Alterloth]

...being blocked on your side from something stronger...or attacking and being blocking from something stronger on your opponents side...

[Arix]

Yes, you can. As I said earlier, there's a moment after blockers are declared but before damage is dealt where you get priority.