Wizard and rogue

[Shinobi]

The campaign I'm running involves the players taking the place of nobles...
At the start 3 of the group were the ranking nobles of a rich county in a large state.

Two of them decided they didn't like the third one, so they gave me a plan how to kill the third player character...

First, they aranged him to kill his wife's parents, so he would get their money and estates. To help in that they gave him as a birthday gift a special enchanted knife with which to kill them. They even aranged to cast a few spells on him, to help him...

A day after he killed them, they captured and took the place of an inteligence operative in the area they knew worked for their enemies. The rogue disguised as the agent and told 3 of his operatives to kill each of the three nobles. Including them. He gave them manor and castle plans, holes in the defense. The time the guards change their shifts and money to buy potions of invisibility. Only the plans for the third nobles castle were right. And to the third assasin they gave a necklase of fireballs knowing that a fireball directed at the assasin would detonate them and knowing that the third noble was a sorcerer that preffered to use fireball, even in small closed rooms.
Paralel they placed evidence in the murdered parents house pointing to the noble, and knowing that he still has the murder weapon on him. That was to discurage any special atempts to find exactly who ordered his death, because he would have been considered dishonorable...

At the end, the third assasin exploded taking the noble and his wife with him. The nobles death was thought to be an attack by their oposition, giving them bigger support against them from the country. They got his 1/3 of the land, plus as his succesors the money and estates of his wifes parents.

Besides that, these two players are known for killing any PCs they don't like. And, I have the problem of finding any player that could match them in their schemes...
Leaving me with only two, but excellent players...