It is the event "a destructible dies", not "a unit dies"
I follow you. You put a Destructible in it, or a Destructible variable. Just press it, then click on any destructible on the map. Or, make a variable, and assign it to a destructo you create through triggers.
.No he wants a unit.
What I would do for this is make a dummy unit with the sapper's Kaboom! ability and make it so that when a barrel dies, the dummy is created and kabooms itself.