The legal principle is simple: it make a person legally dead and so the intentional act is murder. Unfortunately the law doesn't recognise the concept that cryogenic preservation may not in effect kill a person in the long run.

On the other hand, if it were not regarded as killing, you may find healthy people being preserved based on oversold promises of revival from cryonics practitioners.

Perhaps the best compromise for the time being is to regard it as euthanasia, and legalise that with some sensible safeguards. I wonder whether in any countries where euthanasia is legal, opting for direct cryopreservation would be permissible.