Is this being toxic ? How do I deal with it ?

Hello, First of all, here is a bit of context : I met a guy through ranked matchmaking. I was the support and he was the adc. Great synergy, no toxicity. I decided to add him to play together, which we did. But over a few games, he started to be negative. **Not flaming** but implementing a negative ambiance to the game such as saying "ff 15" 6 min in when two kills were given to the enemy team. He kept saying stuff like this. At some point, I had to mute him in game because I couldn't deal with the toxicity. I told him it was unecessary to tilt that early in a game because we never know what can happen. We won most of our games. He replied that he isn't toxic but he enjoys for the sake of it, to be negative and witness his own team tilt or blame each other. I'd like to be able to play with this guy because we have a good synergy in game but I can't handle his behavior. I hate being surrounded by negative vibes but at the same time, I'm done playing solo... And I obviously don't want to be obliged to mute him in game. Is this being toxic ? Will he ever be punished ? What can I do ?
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