On the base level, it is not very hard to examine a chat log and glean who is being blatantly toxic from the log.
A big problem with games like League of Legends or Dota 2 is that you can easily be toxic or cause your teammates to be toxic without chatting or using voice comms.
There are very common trolling methods that do not require any use of chat with the express purpose of trying to incite toxicity in other players, some blatant, and some not.
However, the bigger problem is that honest mistakes can be misinterpreted by your teammates as toxicity:
Losing a close 1v1 vs your laning opponent
vs
Accidentally going too deep into enemy territory and dying once.
vs
Getting killed while attempting to secure map control for your team.
Vs
Playing too aggressively and overextending and dying many times over the course of a game.
When things like this happen, your own teammates may become upset at your poor performance and begin to lash out.
The biggest problem here is that in these games, it can feel like you have no agency over the outcome of the game when your teammates do not perform at the perceived skill level you have of them.
This is where toxic players become hard to deal with. They will start doing things that will incite toxicity in their teammates while maintaining plauisble deniability:
- Confusing teammates by providing useless or inaccurate information about the current gamestate. (Pinging, map calls, cooldowns, timers, etc - many of these require no use of chat of voice comms)
- Picking on teammates by making consistently selfish plays to their detriment.(Courier stealing, going out of one's way to steal farm from a lower position teammate, unnecessary kill stealing)
- Improper role identification, your team strategically expects you to do X, you do Y. Y could even be better than X in terms of winning the game, it doesn't matter.
All of these above examples can either be common gameplay mistakes or intentionally malicious, but the point is that once your teammates do not trust one another, some will start verbally abusing, while others will begin to make similar mistakes as above (tilting) and lose the game for their own team.
Many players want to feel like they were the influencing factor that decided the game's outcome, and make choices that increase the influence they have on the game even if it might actually lower the chances that they win - and this is what many times leads to toxicity.