You flushed them? Please tell me you humanely euthanised them BEFORE sending them down the toilet....
Cervical dislocation would have been an excellent, fast way. CO2 is also a fast, kind death. Hell, even the cats would probably have been faster. You could even have put them outside and let nature take its course as predators would have found them pretty quick.
They were in the paper bag because of you in the first place. How could not *occur* to you that drowning would be actively causing suffering?
You know what, don't even answer that. The idea of drowning a live animal is beyond repugnant.