I am considering demolishing it or selling it for pieces. Am I forced by law to tow it back to Switzerland, or can I (legally) just return the plates and dispose of it in the other country (in EU)?
You brought the car abroad tax and duty free as a tourists and have the obligation to remove it.
You can formally import it into the other country but would have to pay the applicable tax and duty. If stored on private property you can remove the plates and to not forget the gray card. Depending were it is, it can be cheaper to bring it back to Switzerland.
Just happened to a friend of mine. He went to the UK for a week and had his Land Rover Defender stolen from the hotel car park. They basically stripped it leaving the chassis, wheels and engine. It had to be bought back to Switzerland otherwise he would have had to pay import duty, vat etc. His insurance arranged it all and he flew back.