Because that isn't what happened...
I'm really not sure where the concept of Hydaelyn acting maliciously comes from... It has been stressed at great lengths that Hydaelyn is weak, she couldn't talk to us following Praetorium and she couldn't even spirit Minfilia away without hijacking whatever Y'shtola was casting. I see nothing that suggests Hydaelyn wanted to eat Minfilia, I get the distinct impression that she wanted to tell Minfilia what WotM told us, and then wait for us to come collect her as we attempted to, except she made a mistake and was too weak to even talk to Minfilia directly when she arrived... Do you really think Hydaelyn was just giving Minfilia the silent treatment at that point, so she'd get distressed and opt to willingly sacrifice herself? All so she could fail to deliver a message to us and cut off mid sentence? No, Hydaelyn has been displayed as fairly incompetent at this point... What happened to Minfilia is an unfortunate consequence of that IMO. You honestly might as well be complaining that we killed Haurchefant IMO... Hydaelyn didn't check how many talk minutes she had left, and we didn't check our flank...
It honestly makes no sense to me... We were obviously going to come looking for Minfilia, if Hydaelyn is secretly bad and aiming to eat all the XIV waifus, then that was perhaps the dumbest plan to manipulate us with... That it calls her character into question is fairly telling IMO... Either she is much more manipulative and devious than I expect, or she is just an unfortunate victim in all this... I'd go with the later because, well... Look at what the Ascians do for a living... you think the plot is going to turn around and explain Hydaelyn is actually super evil because one person sacrificed herself for her, and the Ascians are justified in the countless lives they've ruined or worse through several Rejoinings? No, Hydaelyn is just incompetent and facing a threat she can't handle...
Gaius actually seemed fairly reasonable in his attempt on Eorzea... He sent an ultimatum, he could have just flown into the city-states with Ultima and taken over, but he opted to negotiate after having just solved our Primal problem... The Eorzean Alliance was even ready to accept the terms until Minfilia walked in and convinced everyone war was a better alternative than Garlean rule... A war that ended in the destruction of Ultima Weapon and the release and subsequent resummoning of Ifrit, Garuda, and Titan... Not to mention Hydaelyn having to exert the last of her strength defending us from Ultima... Garlean occupation is really going to have to be displayed as hellish in order to justify the current narrative... "but we want to worship Nald'Thal and let corrupt individuals attempt to assassinate the Sultanate!" is a piss poor justification for letting the Primals run amok again... I really don't get why people are so scared of Garlean rule, when groups can summon monsters that can steal your free will... I guess there was a scene with Garleans brainwashing children in 1.0 that might still be true (I'd hope not), but at least you can get on Garlemalds good side through service, you think Ifrits tempered get nice magitek toilets and air conditioning for their servitude?