Flushing is only cruel if the fish is still alive. Granted, this fish IS still alive so I wouldn’t flush it, but I see newbs being looked down upon by aquarists all the time because they “flush” a fish. If the fish is dead, it’s dead. Some of us do not carry emotional links to our fish, so therefore to flush him/her doesn’t affect us.
So when a small fish of mine dies, you bet I flush him/her. If it’s a larger fish that typically lives longer, I will bury the fish on my property. But if the fish is alive, I use the freezer method. You’ll get people try to preach to you wherever you go about how to “properly” euthanize a fish but do what you feel is best. Know, however, that flushing a live fish is not considered humane.