The multiplicity of I am

In Vedanta it is said that Brahman is one.

How is this possible?

We have the murderers and the saints, the good and the bad.

If we are all really Brahman and Brahman is one why this difference.

Is Brahman the same in Hitler and Gandhi?

Here is what they actually mean by the singularity of I am

In school, I was taught in chemistry class that water is the same whether it is from a pond or from your tap or from a puddle.

On the face of it, it appears to be a ridiculous statement. Water from the tap is clean from the puddle is muddy and water from a pond is green.

Clearly they are as different as different can be.

Well the chemist would answer. Water is the same, the differences are only due to things that are not water.

And so it is. The same water becomes the tap water and the sewage water, despite being the very same substance.

In the purest essence there is not even an atom of difference between Hitler and Gandhi.