The Borg queen was innocent. Sure, she had a few marks on her rap sheet after that whole "assimilation of Earth" venture, but she changed her ways.
Can you imagine the job stress she has to deal with? And what does Janeway do? First, she tries to take a transwarp coil WITHOUT paying for it. Later she encourags insurrection, civil war, and anarchy among the collective, fracturing them forever. And finally, when BQ was minding her own business inside HER OWN nebula, she spots Voyager, gets nervous, and politely asks them to leave her alone. Janeway then trespasses into BQ's nebula to steal passage through one of her conduits. And this time there are two Janeways. One to sneak through the wormhole and blow it up, the other to infect the Borg with a plague, annihilating them.
Add to this the multiple stolen drones, and there can only be one conclusion. Starfleet sent Janeway to the Delta Quadrant to conduct a campaign of byzantine warfare against the Borg. She didn't have to be asked, she was the first to volunteer.