Greyout (HeiKazu detective/assassin fanfic)

Kaedehara Kazuha leads the life of an assassin taking out the monsters of the world, avenging those who have been harmed in terrible ways.

Detective Shikanoin Heizou condemns violence and fights crimes with only his wit and intuition as his strongest weapons.

Their paths cross when they're after the same criminals - standing on opposite sides of the law each of them follows a rather different approach.
Despite their initial animosity their encounters lead to a deeper connection, challenging their beliefs about morality and justice.
They discover that the lines between good and evil are easily blurred and that nothing is simply black and white but a complex palette of greys.