I've known a few, but there is one that sticks with me the most.
When I was 13 years old, I knew this one 17 year old kid who interacted with my friend quite a lot.
For some background: The kid in question was an orphan who grew up with physically abusive parents, and ended up in the care of his older sister as his legal guardian, who was also cruel and verbally abusive. She made it known to him that she would disown him and kick him out on his 18th birthday. He was very poor, close to impoverished, and his living conditions at home were fairly inhumane, staying on the couch in the living room and also being given pretty small amounts of food. He was a stick, hollowed out both physically and mentally. He also had a social anxiety disorder and depression.
Anyway, he ends up developing feelings for my friend's girlfriend at the time. The girlfriend tells him to piss off, but he keeps doing things to interact with her in subtle ways, like giving gifts (drawings and flowers), writing letters, etc..
My friend is pissed by this, and he informs me of this kid, and asks me to come with as a +1 in order to confront him and shake him down. I do so, and we rather aggressively tell him to cut it out. Frankly, we were quite demeaning and dehumanizing about it. We were kids and took it very personally.
He tries to defend himself by talking about his home life, his suicidality and struggles, that he's sorry and it's the only hope he has left... I pulled him aside and explained to him why that girl will never love him, and that he has to cut it out. He rather meekly accepts it, says he won't do it again... Come a few weeks later, news hit us that he killed himself via hanging. Nobody missed him, and we even laughed about it. Now that I am older and wiser, looking back on it doesn't make me feel sad, but it makes me feel a bit disappointed in myself, since I view our behaviors as having been ignorant, impulsive and, frankly, stupid and emotionally charged. It was not rational at all. We should have just ignored him.
I think the kid was a lost cause; he had failing grades and no employable skills, as well as being a sheltered shut-in. He'd have to be taught how to integrate into the world from the ground up, and for what? No one had any reason or heart to help him. He often refused to permit concern from others, as well as having a lot of misogynistic beliefs and degenerate habits; he certainly was no saint. Even so, I do not blame him, and I believe I should have been easier on him. It was not that serious.