It doesn't seem too wild to use irrational belief in something (faith) to make oneself happier through tough times.
It does seem wild to spend your final days in a pit alone until you starve/dehydrate to death painfully.
These monks aren’t impulsively killing themselves, they seem to be towards the end of their lives anyway and are choosing how they want to go out. I don’t see what’s wrong with that. I might prefer to be in hospice hopped up on drugs when I pass, but I won’t claim to be morally superior for it.