If--and it's a big if--mafia gets two kills (which might fit with Disc being BP but not with there being no NK last night), then a bomb might make sense, I guess. 1-to-1 trades hurt mafia more than town, because mafia is smaller. In such a scenario, town would lose 1/9th of their number, whereas mafia loses 1/3rd of theirs.
Let's play this out, assuming the worst case scenario for town.
DP1: 9-3
---w/ a mislynch and two mafia kills =
DP2: 6-3
---w/ a mislynch and two mafia kills =
DP3: 3-3
---mafia wins
Now, contrast that with a bomb resulting in a mafia death.
DP1: 9-3
---w/ a mislynch and two mafia kills =
DP2: 6-3
---w/ a mislynch and bomb event =
DP3: 4-2
---w/ a mislynch and two mafia kills =
DP4: 1-2
---mafia wins
Only in this scenario could I see a bomb being balanced, esp. with two protective roles in the game. If mafia has two kills, which I doubt, then town would need the protective roles and the bomb. But even so, the second set-up is still questionably balanced. But we need to remember, that even with two killing roles, both of them might not be mafia. So, there are a lot of assumptions built into this analysis.