It starts with murdering everyone in your home village, including a young mother and her husband, though Danette and Lady Lanya survive. After every battle, a not-being-redeemed-in-the-slightest Gig possesses your body for some multiple of eleven days, likely increasing the bloodshed.
As the story progresses, everyone you forcibly conscript into your army goes insane, with the chivalrous Thorndyke becoming a Blood Knight considered one of the saddest changes.
When you finally meet Trish, she's a Broken Bird, implied to be because Hawthorne already raped her by then. And she dies a few chapters later, breaking her sister's spirit completely.
There is another, possibly more disturbing explanation for Trish's mental state by the time you reach her. It's revealed that Hawthorne not only rapes the girls he raises, he kills them afterwards. It is very possible that he does so using poison to make it look more like natural causes. Perhaps, by the time you reach her, the poison has already destroyed her mind and is killing her body as well.
Laharl:Do you have any idea how hard it was for me to get that title?
By the end of it all, Lobo even refuses to work for you any longer, almost every other villain in the game has joined forces with the surviving protagonists to take you down, and winning the final battle leads to you devouring Gig's soul and completely annihilating Haephnes, Drazil, and yourself. And Revya's motivation for doing all this? Probably the same reason the player picked this path: because it seemed fun.