1) He knew the killer's identity, AND was trying to take the picture away from the killer, because he also knew its great value.
The scene with the killer blocking the door of the stable was sorta cheating. This person was able to follow the cousin there, back up a vehicle and place a rock on the gas pedal, all while not being seen. Yet Crossfield , who heard nothing, immediately caught Miss Marple who was quietly looking in a window?
There was also another hole in the story. How did Miss Marple get the painting? Also, why would the nephew allow her to send a painting out from the Inn without at least looking at it? She didn't arrive with one!
The truth of life has been revealed.