I think you may be in the minority, OP, in regards to relating with his character. In general, I've never seen this community rally behind a character more than Emet in regards to a villain. No one was offering sympathy to Nidhogg in losing his sister. I didn't see people posting about how sad they were for Fordola (much to my dismay... she is probably my fave char in the game now
). I always see people pointing to Emet as being a very well rounded character.
However, I do agree with you. I felt terrible for Emet, but I never once felt it justified any of his actions. I think the writers are incredible at persuading the player to become emotional, and leave logic at the door. If you look at this logically, honestly, Emet is a tantraming child someone who cannot move past his grief and thus is very destructive in his melancholy.
Spoilers:
It is heavily implied that the ancients caused their own destruction due to overuse of creation magic. They were arrogant in not helping stop the catastrophe before it became so terrible they had mass killings of their own people. Honestly, the ancients failed. They grew too big for their own breeches. Imo, they fought their fight, lost, and offered a beginning to the next generation. I'm totally on the side of the 14th member. That doesn't make their loss any less tragic or as if they deserved their entire civilization to fall. Just stating from a logical point of view, it makes sense logically why their society failed looking back at the series of events that led upto. This fall was preventable.
The ascians, including Emet, always just seemed like the sore loser. The ascians could have brought incredible good in the world, but instead they just kept hanging onto the fact they lost and just wanted to ruin it for everyone else. I chalk this upto the fact they were tempered by Zodiark, and yes they wanted to save their people, but they just couldn't accept loss.
I think Emet is an emotional character. Yes, we can all sympathize and even empathize with him, but that doesn't mean he's right or his character isn't a mass murdering monster. So, I agree, the character could have had more backstory instead of saving it until the end for the big reveal - but he was the villain. Ardbro was so well developed b/c he's the secondary protagonist. Of course he'll have more writing and development. That's not bad writing, that's just the nature of the characters and how the writer wanted to use them.
Either way, it was extremely effective, almost to a frightening level. Seeing some posts agree with any form of mass murder in this story is really kinda of offputting. So, the writer did get what they wanted. I wouldn't call that bad writing at all.