"There's something in your way
And now someone is gonna pay
And if you can't get what you want
Well, it's all because of me"
--The Offspring, "You're Gonna Go Far, Kid"
So "justice", the ideal the Thief King wants to debunk as just self-justification on the Pharaoh's part, is one part of the ancient Egyptian concept known as "ma'at" which basically, society ran on. By declaring herself an enemy of the Pharaoh's ideal of justice, Papyrus has declared war on everything that makes society society. Everything that separates civilization from the wilderness, truth from lie, order from chaos, yadda yadda, you get the duality.
And really, she's…quite cool with that.
In fact, it's really fucking fun.
She isn't exactly naive about anything, but there's still a feral joy to her that's slowly (and then not-so-slowly) replaced by madness in canon. Even as she throws all of life to the side she embraces herself and her own possibilities.
Does she grapple with guilt about this once she revisits Kul Elna and gets a refresher of why she believes the things she believes? Absolutely. But robbing the Pharaoh's tomb is going to be, once it occurs, both the ultimate slap in the face to the monarchy and also the ultimate jubilation of her own potential and possibilities. She empties the tomb completely to make a statement, but also just because she can.
In short, ROW ROW FIGHT THE POWAH.