It only gets worse for his son; now his mom and grandfather are dead, and his father in jail for his part in that.
"There's more to life than a little money, you know. Don't you know that? And here ya are, and it's a beautiful day. Well, I just don't understand it."
"I just don't understand it."- Marge keeps a stoic expression as she says it, but there's a hint of grief in it. Likewise and before it, how she asks "doncha know that?" to Gaear—who's remained stony-faced to the line-preceding it—in such genuine, saddened incomprehension at the things Gaear does.
The feeling that the murder could be over a billion dollars and she'd still feel the same about this case.
Marge's final expression at the end of the scene reveals that although she had faced so much brutal police cases, she still feels grief for such existence of evil.
The sad realization that such similar and simple statements could be told to the worst of criminals, and that those words would still not mean a damn thing to them. That's why we can't comprehend such evil indeed.