You become a leftist when you mistake the words you and your leaders are using for reality. When you believe wholeheartedly that to talk about improving the world necessarily means that the policies you implement will in fact achieve that end.
I would compare it to the scene in “The Nuremburg Trials” when Spencer Tracy’s character is speaking with an old judge, who had been complicit in the Nazi regime, and long supported them in the abuse of the judicial system for the eradication of political opponents.
The judge, played by Gregory Peck, if memory serves, asks Tracy: “How could I have known it would lead to this? To all this slaughter and destruction?” To which Tracy replies: “you knew the first time you sentenced an innocent man”.
Likewise, the first time you fail to consider the effects of your policy on all people, and across time–the first time you fail to learn from the mistakes of the past–you show your LACK of genuine humanity and compassion, and your consuming desire to surrender your freedom to the automatism of a cult.
That is when liberals become leftists; and most self declared “liberals” are in fact leftists. That is my default presumption, and most of the time prolonged dialogues prove me right. They can’t defend themselves, which means they rejected the need for understanding long ago.