As I was thinking, I came up with something. It's only my own interpretation tho.
I know it's not that clear but the lesson that "INUYASHA" wanted to relay was...

DO NOT BE SELFISH

The legend said that this sacred jewel could grant any wishes. Anyone could wish with no exception. It didn't matter if your wish was for good or bad things. This jewel would never judge.

However, in the series, you could clearly notice that whoever used this jewel for their own good, its pureness would be gone.

It's not as big as killing or murdering but greediness or selfishness is still part of the darkness. So if you wished to that jewel by yourself to grant you power or something, obviously some parts of it would be corrupted.

On top of that, it was already decided that there was only one correct wish to that jewel. The one wish that would make this jewel neither pure nor evil was..

DISAPPEAR FOREVER!!!

Besides, the jewel itself asked Kagome what was her wish when she was trapped in the darkness in that last episode. It asked if she wished to go back to her time, then it would be granted. Later, it asked if she wished to see Inuyasha.

But what demons said was contradicting. They said that if Kagome wished something, anything, bad or good, she would be trapped forever and could never get out. She was scared of that darkness so of course she wanted to save herself so the jewel took advantage of that.

I also thought that it was only darkness because Kagome was afraid but when she said she believed that Inuyasha would come to save her, the light opened.

Inuyasha might have trapped there also if she wished something. Therefore, she wished that one wish to save everyone, not only herself.
TO SAVE THEM FROM THE DARKNESS OF THEIR HEARTS.

That's only my theory tho.
What do you think? I mean if I missed something, share it.
To those who already watched this, let me know what yall think.
For those who don't, then watch INUYASHA

Do you get what I meant? Haha I'm really bad at explaining