How do I make a creature set fire to other creatures and the player when it dies?

You can use a "When entity dies" procedure that says 'For each entity as Entity iterator at x, y, z, in square cube with size #' > 'Set Entity iterator on fire for # seconds'.

Once your entity dies, all entities within range will be set on fire for the designated length of time.

Do note that this would set every kind of entity on fire, however. Including items and xp orbs.

I suggest adding an if statement checking to see if the Entity iterator is '(sub)type of Mob entity' before allowing them to be set on fire. If you still wanted items to be set on fire, though, for example, you could check if the entity is a Mob or an Item.