Karsus was mad, by the end, and was certainly power hungry, but he was well aware of the danger the Phaerimm posed and was seeking to eradicate them or break their power, so he may well have tried the spell that doomed him for such reasons.

Fairly accurate. What I said earlier about the lore on Gargauth applies; this video too often uses the phrase “the greatest” or “the most important.” According to Elminster: “ ’Tis best to usually consider such phrases propaganda.” (Netheril wasn't the most powerful.) ...