The same could be said about how people live their life in general.  Since I don't believe in psychic powers I don't think we can really read a person's motivation unless they tell us what it is (and even then they could be lying - a seemingly devout Christian might have terrible undisclosed private motivations and sin).  Typically nobody probes into the motives of a doctor trying to rescue a patient nor accuses him of trying to achieve a goal forbidden by God, at least not in this day and age.  And typically I think Christians would encourage everyone to have compassion for the sick person who wants to live longer rather than encouraging him to question and second-guess his motives.

Even when a non-Christian doctor saves a non-Christian patient the average Christian doesn't speak out against the act of saving the life - even if both doctor and patient absolutely do want to live a life in conflict with Christianity.

I personally think that one day cryonics will be viewed by Christians just like any other life-saving technology.  Sure it can be used for good or evil, but why should evil be automatically be presumed?  It's not inherent in the technology.

By the way, it's "cryonics," not "cryogenics."  Most scientists who practice cryobiology don't actually want to be associated with cryonicists.