What is a good way to show sad backstory without using his sad past as a excuses for his villain actions and behaviors? Read the description first before typing

Disclaimer: This story is not a romance, nor is it a dark romance. These two characters are not a couple, and I'm not planning on making them a couple.

This story is supposed to be a realistic showing, even though it takes place in a fantasy genre, showing a one-sided, obsessed, codependent friendship between a guy and a woman. Also, this is not torture p\\\\\\\*rn with a female character.

This is supposed to be a dark fantasy villain backstory playing out in real time during the story.

The question I'm asking what is a good way to show his sad backstory without using his sad past as a excuses for is villain actions in the story

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Plot in a nutshell:

I'm got a half faerie man in his early 20s who starts out as the most silent, reserved, soft-spoken, very insecure, and socially awkward toward most people. He is madly obsessed in "love" (this love is one-sided, of course) with his only childhood friend,

The question what is good way to show a villain backstory without using sad backstory as a excuses

For his villain actions and behaviors.

Also I'm not asking you to write my story for me I'm pure asking for advice and suggestions on writing.