I get some variation of this question a lot, and understandably. The idea of Tartarus, of people suffering for all eternity, is very disturbing. It should bug people. But this was addressed in the book here:

“We also use it (The Lethe) on people who’ve done bad things in life,” Cassandra continued. “We take away all their memories, and they serve in the palace or around the Underworld until their sentence is up.”

That didn’t seem like much of a punishment. “Why?”

“For most people, their circumstances contributed to whatever crime they committed. This gives them a blank slate. When they finish their sentence they can live the rest of their afterlife in peace. Of course it doesn’t work like that for everyone, but between me and Moirae we can usually tell who should go straight to Tartarus.”

Anyone can choose to drink from the Lethe, but some people are sentenced to it. Some people are just evil. Not because of something that happened to them or was done to them, but because they just are. Those people still get the option of the Lethe, but because they’d be a danger to the other souls and they need a more severe punishment, are still sent to Tartarus.

I completely get why this is bothersome to a lot of people. But I did base this world off Greek mythology and Tartarus was absolutely a factor that can’t be ignored.