Edit
The bad end in Choujigen Game Neptune Mk2, known as the Conquest Ending.
The CPUs find that the only way to defeat the revived Deity of Sin Magiquone/Arfoire is to use an evil sword which grows in power when it kills one of CPU lineage. Noire, Uni, Blanc, Rom, Ram, Vert and Neptune all reluctantly sacrifice their own lives so Nepgear can use the sword and save Gamindustri. She defeats Magiquone/Arfoire, but the latter taunts her that a world with only one CPU - without strife, without contest - will eventually stagnate and rot, and when that happens, she will return.
The remake, Hyperdimension Neptunia Re;Birth 2: Sisters Generation features an alternate "good" version called the Holy Sword Ending, which splits from the Conquest Ending partway through and leads to a good end.
In Nepnep Connect: Chaos Chanpuru, an alternate version of Nepgear, named Purple Sister V, was introduced, who is heavily implied to be a future version of Nepgear from this route.
1