David Marks killed his wife.....right? What did she show him in the latter part of the film......it looked like a bloody rag? What did his wife see in David's files? Some crooked deals in real estate?
He tells her the dog is at the kennel. She goes to take the trash out & sees the bloody shovel and a rag that may have been part of a dog bed, or something. But it's very clear she knows the dog is dead. He kills her that night.
"I'm with the Government and I'm here to help you." me, every d@mn day of my life
The bloody thing she finds is the blue bandana she tied around the dog's neck earlier in the movie.
Also,I think the money was money they were taking from businesses that were doing illegal practice. They were charging them money "off the books" to let them continue doing what they were doing.