Who gets kill credit in Frosthaven?
The character who caused the damage, with four exceptions: Wound, Bane, another monster, and a negative hex the monster walked into itself.
When a monster dies, the character whose damage killed it gets credit for battle goals, personal quests and anything else that checks for a kill.
The rulebook lists the cases that give no credit: damage from Wound, damage from Bane, damage from another monster, and a negative hex the monster entered through its own movement.
Retaliate is not on that list, which is why a monster that kills itself on your retaliate still counts as killed by you.
“If a monster dies from damage caused by wound, bane, another monster, or a negative hex due to voluntary movement, no kill credit is given.”
Rules change with errata and with digital patches. If your copy of the book disagrees, the book wins — the official Frosthaven FAQ is the next place to look.