The most recent consensus is that freezing is very painful and inhumane. Ice crystals form in the blood/cells.
The mind is the last to go (was reading this on reptiles) and they are conscious of what is happening while the rest of the body shuts down first. Im not entirely sure the difference with humans freezing and ectotherms, but maybe someone can find some sciencey articles on that.
Want to put a fish down?
Clove oil is an easy way to put a fish to sleep. Or quick physical trauma- get the brain/spine severed, crushed, destroyed using force. Its downright 'dirty' so it is easier for people to put a fish in a freezer and not worry about it vs using your own hands to end a life right there.