The game will check to see if a creature has died that turn when Brimstone Volley resolves. This is because this specific morbid ability is a static ability that generates a replacement effect. This replacement effect is applied as Brimstone Volley resolves. (You can tell this is a replacement effect as it uses the word ‘instead’.) It does not matter if a creature has died when Brimstone Volley was played. This means that you can play Brimstone Volley, and then cause a creature to die before the Volley resolves in order to have the Volley deal five points of damage.

The morbid ability of Caravan Vigil works in a similar manner.