Encouraging toxicity in any form crosses the line (IE "egging" them onn). You don't need to brag about anything. Let the scoreboard speak for you. If you won your lane, good for you. You don't need to rub it in further. If you ask me, I'd say yes, it's toxic behavior specifically because you're doing it to get a predictable reaction out of them, and that really isn't right.
If it were up to me, I'd remove all-chat from the game. With the exception of some people able to keep discussions civil through all-chat, most of the time it just ends up being used in a toxic manner, and doesn't add anything to the game. If you want to talk to people, or brag about something, do it to your own team.