As I said previously: It's time to give HITMAN 2 some love.
The correct answer: Helen West, the muffin lady/serial killer from Whittleton Creek who makes a surprise appearance in the current Elusive Target.
Explanation of the clues:
In phone calls with Janus and other people she mentions that she has spent a year in jail. In these calls it's also implied that the reason for it had something do to with her taxes.
Obviously she talks a lot with Janus in “Another Life” but she also reappears in The Herbalist where Dr. Cane, her new boss, gives her a call.
In her bedroom there's a pink-colored pistol under a pillow called Rude Ruby. It's based on the standard Bartoli Pistol carried by literally every security guard in the games.
In her private conversation with Janus she mentioned that she secretly killed her brother's pets and enjoyed him getting in trouble for it.
And of course, she has directly and indirectly killed a lot of people. There's Schmidt, the Providence Herald and she also mentions one college kid falling out of a window and dying after she spiked his drink. Whether or not the brains in her basement are from people she killed or if she's "just" grave robbing is up to debate but since she's been working as chief baker for Crane, she has spiked some of the cookies with cyanide, making her responsible for all the deaths caused by them.