Don't know if anyone needs an opinion of a fairly new member, but here are my two cents to this topic:
The way I see it is that the reason you have to defeat the wall of flesh and moon lord is because, while things seem fine as they are, the moon lord is slowly regaining power, and the corruption/crimson slowly spreading. The Wall of Flesh, seeing that it cannot stop these things from happening (or probably being tired from holding back the ancient spirits), created a human form for itself to search for a new guardian, someone who could stop the corruption and defeat moon lord once and for all, and that would be the player. In turn, the fight with the wall of flesh can be seen as a final test - it needs to know if you're truly ready for what hardmode brings, so you must defeat the hardest enemy available to prove yourself worthy - that would be the wall itself. Once you come out victorious, it unleashes the spirits of dark and light, leaving it mostly up to you to handle it all. The world itself is not happy with losing it's guardian, so it creates the hallow as a "temporary solution" (the hollow itself can be seen as not any better than the evil biomes - seeking to consume all "impurity" (which is everything except the hallow), and having a tyrant fairy goddess doesn't really scream "true good" to me).
As for the pillars and moon lord - yes, releasing him might seem wrong at first, that is until you realise that if you don't fight him as soon as possible, he may regain more power and come back too strong to handle.
I am not very familiar with the lore, I have done quite a bit of reading and searching, but haven't really "dug deep" into it, so to say. So, this might be extremely controversial to the little bits of official lore we have - this is just a hot take, please take it with a grain of salt.
: