There are likely a few things at work here.
The first is that if you were immediately banned for 14 days your logs probably involved behavior that Riot has deemed Zero Tolerance behavior. Without looking at your chat logs we can't know for sure.
Second is that you do not always get an alert when someone you punished is banned, and they are not always immediately punished after a single game. Sometimes the system is waiting for more information to confirm their behavior, and sometimes they just haven't accrued enough consistently negative behavior to trigger a punishment.
Thirdly, you are always judged only on your own behavior. When the options to simply mute the other player or not respond to them exist, any decision you make to engage in a toxic manner is entirely on you.