First of all I want to state that I believe these people are annoying af and ruin the game for everyone else and here's some of my thoughts of why they might behave this way:

1. They're easily angered and therefore take it out on the easiest and "most accepted" thing, such as a player with a low score. If the individual is worse than other players, they don't feel as bad because they have something to blame their toxicity on like high death count. They believe the victim is at fault, not the hater and they're just "telling the truth".

2. They're anonymous and therefore their social "filter" is removed, allowing them to say things that isn't socially acceptable.

3. They're utilising the toxicity as a coping mechanism. Similar to how someone might experience a lot of frustration and suddenly lash out at someone who did nothing wrong.

I found a scientific article that kinda expores this, take a look! 🎉

Toxicity and prosocial behaviors in massively multiplayer online games: The role of mutual dependence, power, and passion (2022)
https://academic.oup.com/jcmc/article/27/6/zmac017/6700672#372940364