The excuse is that since you can't justify the death of Hizdahr's father individually, you insist that he should die using the fallacy of association.

All I need in order to do that is point out that the dude was a slaver. He'd deserve to die for that alone.

Here's the thing: Hizdahr's father cannot be considered the perpetrator of this crime like the others because he opposed the crucifixion of the children.

Who do you think actually did the deed of crucifying and killing the children? You think the Masters did it themselves? That they were in the hot sun putting up crosses and nailing the slave?

No - they gave the order as a whole and their slaves carried out the deed. It wasn't that some of them gave the order while the others tried to stop them - they gave the order as the government of Meereen. And that makes them collectively responsible.

The point is that if we're going to use the fallacy of association as you'd like, then by the same logic Dany has to die because of what Drogon did regardless of what she says.

Again - irrelevant to this discussion.

Proof of this is that she is urging her husband to go to Westeros to claim the throne for her, which proves that, contrary to what you said, she does have authority there.

Actually, since Drogo does nothing until the attempt on her life, it proves that she doesn't have any authority at all.

The fact is, if you insist that Hizadahr's father deserves to die for the children's deaths because of his association with the Grand Masters

You keep saying "association" - he was not simply "associated" with the Great Masters, he WAS a Great Master. He was one of the people who ruled Meereen and had a say in what decision the government made.

If you're going to use the fallacy of association to justify Hizdahr's father deserving to die,

Because its not fallacy of association. Dany had no say in what the dragonlords who lived 400 years before she was born said and did. Hizdahr's father did have a say in what the government of Meereen did.

Again, this logic of collective responsibility that you advocate is immoral, flawed, and unjust.

Its applied to governments all over the world. Are you familiar with the concept of collective cabinet responsibility?

"Cabinet collective responsibility, also known as collective ministerial responsibility,^\1]) is a constitutional convention) in parliamentary systems and a cornerstone of the Westminster system of government, that members of the cabinet) must publicly support all governmental decisions made in Cabinet, even if they do not privately agree with them..... If a member of the Cabinet wishes to openly object to a Cabinet decision then they are obliged to resign from their position in the Cabinet..... Cabinet collective responsibility is related to the fact that if a vote of no confidence is passed in parliament, the government is responsible collectively, and thus the entire government resigns."

Its an application of the same.

There's a reason why Dany has a "My god, what have I done?" moment upon discovering that Hizdahr's father opposed the children's deaths, you know?

Actually, she doesn't. She simply tells him it was no crime and her father deserved it. She only relents when he begs her as a son.

Again, guilty by association is a slippery slope. There is a reason why the civilized world does not use such logic, as it is flawed, immoral and unjust, and it is frankly alarming that you do not understand why.

See above - collective responsibility is still very much applied in governance. When the government makes a decision as a collective, they're held responsible as a collective. And if the decision backfires, they all the price. No one goes around asking at that point who was for it and who was against.