That is true.

Soul gem itself is the soul of a magical girl.

Eh..?

Madoka.

That is correct.

If the soul gem turns black, the heart also turns black, and the fate that lies at the end of it is... remains the same.

The "anti-witching" ability expressed in Sayaka is never "not becoming a witch".

Still, as long as certain conditions are met.

It wouldn't have led to witchcraft, but...

Perhaps it was only an unfortunate fate for Sayaka that you showed up here.