From my limited experience with the dataeditor you'd call an action actor when the building dies, and in that you'd call well, I'm not sure, something which checks AoE around the building and then from there you'd apply damage from an effects actor. I hope this helps as noone else has responded yet :)
In Warcraft 3, I would take the kaboom ability, but in this, I can't find something like that, because baneling does not damage when it dies.
From my limited experience with the dataeditor you'd call an action actor when the building dies, and in that you'd call well, I'm not sure, something which checks AoE around the building and then from there you'd apply damage from an effects actor. I hope this helps as noone else has responded yet :)
Behaviors can specify a Death Response effect, look under the Modification field.
Also, banelings do indeed explode when killed.