Fair question. But unfortunately you’re using historical hindsight to level charges that aren’t actually valid.
Aang didn’t know the Fire Nation were planning a genocidal attack on his people. Aang didn’t know Gyasto was planning on keeping him safe. Aang had no idea that flying out of that window would mean the death of all his people. He didn’t know that a storm would keep him from the world for 100 years.
He was the youngest person ever to be told he was the Avatar. An event that ostracized him from all his friends. He was an outcast, and the only person in the world that stayed closed to him, Gyasto, was going to be torn away from him. So he ran. That’s his character weakness. Aang runs from his problems. And it has disastrous consequenes. But you can’t blame Aang for the Fire Nation committing genocide. Those soldiers are the ones that murdered all the children and peaceful Air Nomads, not Aang.
What makes Aang a great Avatar is he overcame his weakeness. Throughout the series he comes to accept his destiny. This 12 year old guy accepts the fate of the world on his shoulders and faces the embodiment of the ideology that murdered his people and deals with him justly. Aang is a great Avatar because of the circumstances he was presented with. Not only was he unprepared, out of touch the morality of the modern world and dealing with problems 100 years in the making, he did this all at the age of 12. He fucked up back in the past, but that tiny mistake which had huge unimaginable consequences, he was able to get past it and literally save the world. If that doesn’t make a great Avatar, I don’t know what will.