Just to step in and make a few notes. A few years back, I recall the staff directly stating in a war thread that the DF hero is, in no uncertain terms NOT evil. That may have changed since then.
With the Interview Quest you are able to determine your own personal alignment, but this is not solid to all iterations of the Hero.
To put this out, to hopefully quell potential debate on it. The definition of hero is as such: a person who is admired or idealized for courage, outstanding achievements, or noble qualities.
By this definition, the Hero is in fact a hero, having contributed majorly to the defeat of SMUDD and Wargoth, the various quests collecting the orbs, and by simply being admired by many that they protect. The alignment of Good/Neutral/Evil cannot be applied to the title of Hero.
The definition of Good is most applicable when addressing the hero and their actions as a noun. The definition for Good as a noun is as such: that which is morally right; righteousness.
As the hero has a significant tendency to do what they, at any given time, believe is morally right or righteous, they are in fact inclined to do good.
To step back to the intent of this thread and away from this debate.
I do not believe that it would be as simple of a task to replace the Mana Core as "have Warlic take its place" or anything of the sort.
Assuming that the Mana Core operates similar to our planet's molten core, the destruction or destabilization of it could result in cataclysmic shifts in the earth, causing earthquakes, fissures, landslides, and the likes, creating mass death within very short time, depending on the severity of destabilization.
Likewise, creatures such as Dragonkin, Clawkin, and Dwarves who are all heavily reliant on mana and/or magic could die out in mass quantities before any damage could be repaired, if not entire races wiped out, assuming that they need a constant source of ambient mana.
The End of Magic, however, may not be as outright foolish as tampering with the Mana Core, but instead rely on sealing off the ley and limiting the supply of mana to the world.