Well, they simply couldn't do anything else, it was either get clapped by Bernhardt later or risk it all on a final gamble. The people's support doesn't matter much in this situation since the military are already following the supreme leader's order to seize HERO, and if they succeed, they would have enough power to force them into slavery.
In line with what
Dinoflaggelate has said, my guess is that they plan to use Dexter as bait/hostage and the S-class to force Hero to deploy most of their heavy hitters away from their base, and then have a squad infiltrate it to extract Ella's corpse and whatever else they need, with another horde of monsters providing chaos outside. This is probably where we "fight" SIN's walking bag of holding with his new and totally not suicidal arrow.
(It would be funny to see Malik fighting this entity by himself and then a portal appears, the cavalry is here... potato... just a little trolling).
Somewhat on the same topic, in the brothel dead end, Malik still destroys Monster World, so it's quite possible that HERO wins regardless.
Well yeah, he keeps choosing the spawn of the 4th.
Could it be Deus? He gave Valravn his heart back, so he might be planning something for this event too.