If you know somebody with carbon dioxide tanks for a soda fountain, seltzer bottle or home bar you can make a euthanasia chamber that is very humane.
Tape or rubber band a plastic bag to the airhose with the newt placed inside it and open up the regulator to let a good steady flow of CO2 into the bag. Herps can hold their breaths for a long time so leave him in there for at least an hour. The gas will send him to sleep long before levels build up high enough to euthanize.
Otherwise I'd recommend sudden blunt head trauma or decapitation as suggested by corr. Or a very quick, hard snap of the neck to break the spinal cord. Only do that if you have strong hands and use very sudden force though. A partial breakage is probably worse than the freezer.
Good luck, and I am sorry for the loss of your newt.
The Zombie Mama is here!