They are literally gods. Their stats showed that they had the ability to cast the wish spell multiple times each day--without it getting monkey paw'd, because they have the literal gods of magic on their side--in addition to all of there other bazillion spells and godly powers. There is zero chance that a trolley problem of any scale is actually a problem for them.For the utilitarians it comes down to math, which makes it more concrete. According to their view of ethics, it's perfectly justifiable to kill one person to save two, to kill two people to save three, etc. Take a look at the Trolley Problem. A utilitarian would throw the switch and kill one person to save five. Fizban reads like a hardcore utilitarian. So from his own perspective, he's perfectly justified in his actions. He saw that the darkness was closing in and would consume the world forevermore if he did nothing, condemning all future generations to misery, slavery, and eternal crapsack world. So he chucked a mountain at Istar and killed a few million people to save a few billion.
"Oh, I have to choose which people I can save? Well, I guess I'll just cast time stop on myself, save these people here, then teleport over there and save those people, then drop the time stop. Everyone is safe." Or "I save these people, then cast resurrection on those people, then cast restoration to restore the lost Con point."
Which means, in-universe: "I'll cast quest on the kingpriest, ordering him to stop what he's doing and repair the damages he's caused in order to redeem himself in our eyes." You know, quest, the cleric spell specifically designed to get people who go against their god's edicts to change their behavior and atone for their sins. I'm sure being cast by a god means that the spell causes has an enormous penalty to the kingpriest's saving throw. And that's assuming the gods don't just power word kill him. And that's assuming that these gods even had to cast spells--I don't know what sort of powers 1e gods actually had beyond their spellcasting, but they may not have had to cast anything. They maybe could have just willed it into happening.
But if they can drop a mountain on millions of people, then they can also cast a spell pinpointing only the ringleader.
These sort of philosophical justifications only work if you're talking about real-world humans who only have human powers. Once you get the ability to manipulate time and space and reality at a whim, those justifications fail to justify anything.
Face it: the gods had a ton of options. The writers either weren't able or chose to not think outside their religious box. That is the only justification--the writers decided it. Which means other writers could decide it differently.