Possible ending for this plot: the betrayal causes either the single mother or the neighbor to kill the other one, and commit suicide.
Who might, from my viewpoint, have the best expertise to handle this plot, again:
1. Lars von Trier (special aptitude, and in-depth exploitation, on such topics; will include a lot of full-frontal scenes)
2. Joel & Ethan Coen (to make the plot slightly farcical and less heavily-themed)
3. Darren Aronofsky (not bad; he can replicate something greater from Requiem for a Dream)
4. David Lynch (may be too surrealist and dark)
5. Atom Egoyan (sorry, again, you’re on bottom of the list; this plot might have a Rotten Tomatoes rating of mere 10%)
How the plot can be improved: the best time setting might be somewhere around 1960s or 1970s, during Cold War’s heightened tensions between United States and Soviet Union. The ‘hitman’, as I want to suggest, might be an illegal immigrant from East Germany, who, being penniless and illiterate in English, struggled to stay alive throughout his time in US, and how he ended up, in a downward spiral, reluctantly joined a White-supremacy organized crime group in Las Vegas (okay, this city might be a bit too mainstream; give me some other possible suggestions please).
Or make it like this: he used to be a millionaire, but ended up poor for his addiction to gambling and drugs throughout the Great Depression. Somehow, unexpectedly, after NAZI rose to power, and persecution of German Jews began, he became a hero, though after the World War II, he met his own tragic ending. You could decide how his life would end, whether he was deported to Siberia or to arid desert in Central Asia. You choose.