Q. Is Source a personality or just a collection of potentials? If it has a personality and the ability to stop evil, then I think Source is either indifferent or cruel!
A. The “Source” you speak of is, I believe, Nirguna Brahman, Brahman unmanifested, and is the singular point to which everything seemingly “real” returns at the end of each cyclic Yuga. At that point, Space no longer exists, therefore neither does Time and without Time there can be no sequential thoughts. So yes, you’re right to think of Source as all potential, as “just is” or pure Being.
Source unmanifested is everything and nothing, without attributes or personality, and as surely as Source exists, then too another Yuga begins with a manifested Universe (Saguna Brahman).
It is only as the Universe (including us) that Source can experience anything. When unmanifested it experiences neither “good” nor “evil’—both are just potentialities—but when manifested, Source has the ability to experience anything through us. Yet it never experiences everything because “everything” is unlimited.
Q. Which came first, Nirguna Brahman or Saguna Brahman?
A. Neither Source unmanifest nor Source manifest came first. They both did. Or neither did, depending on your perspective. Like a circle that has a starting point, but once the two ends are joined it is infinite, the two aspects of Source (unmanifest and manifest) are infinite. Both are the cause and effect of each other. Both are eternal, except when viewed from a temporal perspective, and then any concept of “first” is merely an illusion. Source, regardless of whether it manifests or not, is eternal, which means so are you!