There is some relationship between Madman and Victim that prevents the Victim to react as it would in other case. The very "not responsibly" idea about madness could be what makes the Victim forgive the Madman.
The "madness" is whatever could alter the conscience of reality or right/wrong of Madman: actual madness, temporary derangement, berserk fury, magic enchantments, spirit possession...
Madman is victim of madness, its conscience and/or will is altered.
PCs as Madman
If PCs are the Madman they are victims of the story. For them to still feel like main characters, it has to be a way for them to realize they are crazy and do something to cure it while avoiding damage to their loved ones/innocents.
For that, the madness should be intermittent, such as a berserk fury in really stressful situations or a lycanthropy curse.
The Victim element can grow as the story develops, when more and more characters are wronged and become a new part of the Victim. It also may stay stable in number of characters, while the threat of new victims or new wrongs still exists.
PCs as Victim
The PCs need to know there is madness moving the madman or they will do as with any other offender. It is best if, besides the madness, there is some kind of relationship with the Madman that forces the PCs to look for a cure, to take care of the Madman, etc.
For the story to have a sense of victory, though, they need to reach a solution. Maybe they get a cure for the madness (or curse/enchantment) or even accept the need to sacrifice the Madman (by killing it as well as intern it into a mental hospital or whatever). These solutions, however, are not part of the Madness story but stories developed from it.
Easily the Victim may become the Unfortunate in a Falling Prey of Cruelty/Misfortune, with the Madman as the Master of Misfortune.
If it recovers, the Madman may develop the story in Erroneous Judgement.
If the Madness is long term, finding the cure could require a Daring Enterprise and when the cure is applied it could be Recovery of a lost one (the Madman)
If the Madness is permanent, this can be mixed with the effects of Loss of Loved Ones