The Jax hate is mainly due to fan comics portraying him as a jerk with a heart of gold and then episode 2 came out and the fandom got made their head canon wasn't canon. Despite Gooseworx openly stating Jax was always supposed to be a scumbag long before the episode came out, they accused her of "ruining" the character.
@Hazbinhotelanddcfan Seryu is like the Gabi Braun of her verse. Kills a fan fav and everyone 100% hates her, refusing to see any nuance. I've seen some compare her to Shou Tucker and Griffith for most hated characters
Seryu killed one fan favorite and gets the most hate from extremist AGK Fans. And this is the same verse where Syura, Honest, Champ, Merraid, and Bach exist