While I agree with everyone on here that the snake would probably be best off if it were put down....

Is it morally okay to take an animal, promising to give it a good home when you fully intend to kill it?

Although we think we know what's best for the animal, it's not our call to make right now. It's someone else's pet and they(supposedly) have been caring for it for some time and are making an effort to find someone else to care for it. They are not looking for someone to kill it.

Not that it's not "the right thing to do" to euthanize any animal for quality of life issues... but really, we could extend that to some of the PETA folks who would happily "adopt" free pets from any of us and put them down because "wild animals cannot be happy in captivity".

Just my $0.02.