The best argument against going back in time and assassinating Hitler is that you know how it ended - Germany lost and Hitler blew his brains out. It cost forty, fifty million lives in total but how much may it have cost without Hitler?

Without Hitler, Germany may have remained weakened and unarmed as was intended in the Versailles Treaty. In this environment, and with a policy aim of spreading Communism worldwide, Stalin may have been able to mobilize the Read Army (as Germany mobilized the Whermacht) and sweep aside any opposition in its path (as Germany did in 1939-1942) but the Soviet Union would not have to fight a two-front war against the Soviet Union which chewed up its resources and allowed a knockout punch from the other side. Imagine Red Army troops marching under the Arc d'Triumph rather than German ones as the USSR stretches from the Pacific to the Atlantic. Once the Soviets were in an unassailable position in Europe, the world would be divided in two as colony after colony fell into the Soviet orbit. Imagine just how much blood that would cost.

Stalin managed to kill over thirty million people before WWII even began. As has been said elsewhere, the US armed and mobilized in response to Hitler but maintained those forces to contain the USSR once Germany was defeated.

And that's just Stalin. Who knows who else may have shown up.

Now having been through that experience, we can now recognize (we hope) when it looks as though another Hitler may appear and take steps to contain them.

That said, I would definitely go back and put a bullet in the brain of any effective underling in the service of Hitler once the war had begun (Heydrich comes to mind)