I'm going to pretend to help search for a lost puppy when it's actually in my freezer
Thats ghastly. Why would you kill a dog and put it in a freezer?
Because it was an accident
Accidents happen but you need to come clean with what you did. The owners deserve closure.
I'm going to lie about it. I don't want my girlfriend to get mad.
How would you feel if your dog got lost and someone knew the truth and didn't tell you? Your girlfriend will get even madder when she finds out what you did to the dog. She might even break up with you because putting a dog in a freezer to hide the evidence is absolutely disturbing and morbid.