It is indeed true - the extreme left and right (along with the religious extremists of any religion) all believe that they have the One True Solution, and have historically been ready to murder millions who they believe stood in the way.

So yes, both the extreme left and the extreme right are not only wrong, but morally repugnant.

The whole point of classical political liberalism is of how to get multiple incompatible ideas of what is the good life (broadly understood) to coexist peacefully. Without it there is only tyranny or chaos.