Toph confesses to Zuko that she killed people. She never wanted to, but earthbending is flinging big chunks of rock at the adversary so even if you're careful something is going to be broken. She's a twelve-years-old girl and she killed people, and she didn't even know she did until Jet get mortally hurt and she realized she had busted people just like that.
Sanura's Dark and Troubled Past. Her family, her love and her village have been wiped only because a couple of Goaul'd wanted to make a movie. No wonder she decided to kill them.
When he learns Toph and Zuko's disappearance, Aang gets serious. It's easy to forget, but the kid is a Fish out of Temporal Water who desperately clings to his friends. And now two of those friends have been kidnapped and he doesn't know how to reach and save them.
The Avatar Spirit's reaction to this is also heartwrenching, because it means that the Stargate is reactivated, and it thinks it knows who had Toph and Zuko; it's implied the Avatar Spirit is the result of Playing with Syringes by the Ancients, and it fled with its friends to a safe world. Only now, it's not safe anymore.
SG-1 does not take well the revelation that their new guests are child soldiers. Especially Jack.
Zuko grows sicker and sicker from using his firebending while being away from the Sun. He finally collapses after protecting an infirmary full of Marines from the yukioso, and the soldiers are horrified and ashamed to see a teenage boy in a coma because they weren't able to protect themselves. However, Toph's reaction is the real clincher, because she can feel her friend dying and no one listens to her explaining how to help him.
The simple fact that Djehuty is the only sane Goa'uld to live. He lost hope to save his civilization a long, long time ago.
