Don't flame in post-game chat if you win.
Please show some humility. Most of the times this happens (on my or their team), the victim usually doesn't deserve the flame, and weirdly it often happens to the person that tried their best or was doing well on the losing team. In all honesty, flaming your lane opponent or whoever was an actual threat to you in post-game chat only makes it obvious that you've finally won a game after a long losing streak. Just move on with your victory and for the love of god stop contributing to an already toxic community.
EDIT: Actual constructive criticism is different from straight up flaming btw