When the young Arthur saw his mother underwater. Later he told his dad, but he tells Arthur he was just seeing things. He tells Arthur that his mother left them both and it's just the two of them, and then hugs a saddened Arthur.
When Arthur asks Mera what his mother is like, she tells him that it is time for him to find out of himself. Orm killed her just before that scene. It gets sadder when you remember that Arthur's dad also died shortly before the start of the film.
The death of Stephen Shin, he tries to help Aquaman only to be killed by Atlantean soldiers shortly after they've met.
We see Cyborg lamenting the loss of more of his human parts, thinking he isn't truly human anymore because of it. Shazam tries to cheer him up, but he's too depressed to listen.
Orm, as terrible as he is has a perfectly good reason for hating his mother. She blatantly hated his father, her husband. Cheated on him with a surface dweller, favours the bastard child she had with him over her first born and would rather hand over the throne that has been his right since birth to someone that doesn't even know he's half Atlantean let along the son of royalty than him.