My take on the ending..
Good or bad (I personally liked it) this is what I thought of the ending.
Elliot had been pushed around all his life. And just took it. Abused by his father (which given his situation as a child was not much he could do so no blame on him), tormented in school (he smiled after being beat up by the two brothers), and slinks away after getting fired at work.
Now he is in this game where he is pushed around more, while being reminded about how much he has been pushed around.
Elliot leaving with the money to me would be another instance of him being pushed and not pushing back. By killing the cop he takes matters in his own hands for the first time in a long long time by his choice and nobody elses.
He may have lost the money but he is in no worse of a situation then he was 36 hours before.
"I reject your reality and substitute my own"-Adam Savage