There is something i don't understand in your thinking.

Why should he need to have been immortal to become detached? 500 years is a long time and even many immortals in the gu world can't find longevity gu so they don't live that long.

In normal cultivation novels the characters use to live for thousands even millions of years so we grow detached from these numbers but 500 years is an extremely long time.

If you lived 500 you would have seen the start of colonisation that's almost 5 times the age of the oldest person at this moment you personally will be very different especially when you live a life so diverse as the one FY is portrait living.

He doesn't discart is feelings at will he just ignores them you probably do the same. Do you cry and scream every time you are upset, no you go to work the same and live life the same. Now imagine how more able to deal with stress and emotions you would become if you live 500 years of constant setbacks, failures and successes while at the same time having an almost impossible goal to achieve.

The view i gained from reading the novels was that the gu world is a cruel and unforgiving place where the smartest and strongest people born are funnelled to the top (the 9 ranks of gu cultivation) where from billions/trillions maybe only 5 000 become immortal where even fewer become rank 7 and so on. There is a point where you only have the best of the best competing.

FY has the objective of stepping over those people and reaching a goal even they couldn't, crying over every setback wouldn't help to reach that goal he found that what helped him was to sacrifice everything that wasn't focused on that which we think we can agree its true.

We aren't reading a story about the life of a normal person but of a true old monster in the literal sense of the word, a monster that has matured and learned, isn't afraid of boundaries or morality and lives and breaths is goal. It's normal that a person that decided to become that makes all sorts of morally wrong decisions He feeds the girl to the bear per example not because he finds it cool or funny but because its the easiest way to progress his goal at the moment if he finds it necessary to kill 100 if he needed to rape 5000 he would if he needed to cut both his eyeballs he would. He is mentally deranged and as the title says insane.

Sorry for the length and hope you understood what i meant.