Religious beliefs are an undiagnosed mental illness.
Of course, that conclusion begs further questions. Should unwelcome proselytizing be considered an attempt to do harm? Should it be permissible to indoctrinate children into religion? These are not easy questions to answer without grossly infringing some individuals' rights to free speech and religion.
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.
Religion is basically always a socially acceptable mental illness that we not only tolerated but encouraged because it has been a part of civilization since before we formed the basic premises for what constitutes a civilization.
That doesn’t make it an illness, unless it produces massively negative outcomes.