Including the person being toxic.
Your teammate is not going to magically do better because you're trash-talking them.
The pain of a loss is not going to be eased by last-minute bullying in the end-of-game lobby.
On top of that, both parties are going to forget it in a few hours.
You will gain nothing from being toxic, but you are still liable to loose something; in spite of what the internet may tell you, Riot's punishment system does work a great deal of the time.
So please, resist being toxic for your sake. The person's probably muted you by now, so you might as well be screaming into a hole in the ground.