The loss of a cast member threw the whole cast and creative team a huge monkey wrench. They obviously had episode written for the new year and had brought out some wonderful and funny episodes to the actors for the new season. The loss of one of the main characters presented a problem. How to bring a new doctor to the show and show respect for the loss of their friend.
I thought Monk being way off his game was a fitting way to do it, and it carried through the whole episode. I know when I have lost a close friend or associate at work I am off my game. Monk was off his game because he had no outlet, Natalie tried but she was not what Monk needed.
Monk buying the house on impulse, not seeing the problems of the light, the scuffs on the step, the wallpaper, the mold situation and even the dust showed he was preoccupied.
The reason that Monk and Natalie were not killed is simple, it is one thing to steal the money, but to kill Monk would have brought the wrath of all law enforcement down on them, something he did not want.