My dad did when I was 18. He was a heavy alcoholic, kicked my mom out then kicked me and my sister out before I went back to help him out.
We had a huge fight about something the night before, and the following morning he wanted to make amends and asked if I wanted to do something with him after work.. I was still so angry, I said I wanted to hang with my friends tonight. He looked really defeated then walked back upstairs as I left the house and that was the last coherent conversation we ever had together.
Came home from work+friends and he was lying in bed next to about 20 cans of Schlitz malt liquor and throw up all over his bed/mouth. Called 9-11 and tried to wipe away the stuff clogging his airway, thought I killed him after paper towel got stuck there and he stopped breathing before the paramedics showed up and took him away
Came to find out he got drunk (like he always did) but decided to drink antifreeze and that's how he ended his life. He left a suicide note that was pretty basic about how he was tired of life and bills and who gets what... Then we read his journal and he said some REALLY nasty, mean, and hurtful things about me and my sister. Could have been his way of trying to justify not sticking around for us.
I have a complicated view on him as a person, and it changes depending on the day.. sometimes I feel sad, other times happy he passed, other times angry. It affected me greatly I ended up homeless and had a much harder life post HS.
All this to say.. people have very different reasons for doing it, but the friends/family always suffer the most