My concept for a new class of Cyberman - These Cybermen retain just enough of their humanity to retain individuality.. they have no recollection of ever being human, but the flaw in their conversion means that they have no built in allegiance with anyone, nor do they have any form of moral structure and as a result are exceptionally dangerous,.. they exist only to further their own ends, take what they want, when they want it, and kill / destroy anything that gets in their way, human, Cyber or other, with no second thoughts or remorse. Their individuality shows through in the way they decorate their bodies, painting their armour in a myriad of styles, each different to the next, and in their voices, which retain the robotic tone but with an inflection which tells of their deep set murderous and destructive impulses.
My idea is that the Cybermen take a prison, and when converting the prisoners their conversion machine develops a fault during the conversion of the prisons most psychopathic and criminally insane multiple murderer inmate. He staggers forward from the chamber, stripped of his memories and humanity, retaining only his lunacy and violence and sense of self, turns to the nearest Cyberman and tears off its head, before wiping out the rest of the room.. he then systematically converts the rest of the prison using the faulty conversion chamber, building his own private army..