Why did Phil do this to Howard at the end of “Uncut Gems?”

Question

Just watched Uncut Gems (2019) for the first time and was thrown for a loop about the ending.

The whole film is about Howard Ratner (Adam Sandler) seemingly endlessly gambling in his life: With money, relationships and pretty much anything/everything.

Throughout the film he endlessly loses. But at the end of the film, he bets and wins. The thugs at the end of the film, who are trapped in a security vestibule, see this for themselves. And they know his winning bet means that Howard can finally pay off the debt he owes to them.

But when Howard opens up the vestibule to let the thugs back in, because one assumes everything should be fine now that they all know Howard has the money and the debt can be paid off, Phil flips out and shoots Howard in the head.

Why did this happen? One can assume that Phil just lost his mind and decided he doesn’t care anymore because right after he kills Howard he kills his boss Arno as well for flipping out about Howard being killed.

So why did Phil kill Howard? Was there anything in the film that explains Phils behavior past the fact he’s a violent and impulsive lunatic?

Pictured are Nico and Arno enjoying their unplanned respite in the security vestibule outside of Howard’s jewelry shop.

Answer 1

It was a combination of rage, self defense, and opportunity:

• Phil was enraged by the treatment he had just endured... not just held against his will, but also actively taunted by Howard during the ordeal. Not the type to forgive and forget, he killed Howard in angry retribution.
• Then he killed Arno because Arno was now upset with him and therefore a threat.
• Finally, the store was loaded with valuable items that Phil and Nico proceeded to steal, meaning Phil's killings turned the situation into a much bigger payday for himself.

Answer 2

In addition to Shizs answer, i always assumed that he was brooding on this for the entire movie.

Howard was a loser to him anyway, someone to push down and extort. For the first time Howard was starting to have the upper hand and Phil planned on taking everything for himself.

Arno was always the leader of his group. But in their short entrapment he lost his authority. Howard was in charge of the situation now, not Arno. And Phil did not accept Howard as a superior.

It was his chance to restructure the dynamic, get rid of two people to potentially look up to, take all the gains from the robbery and take over the gang.

He planned it out, waited patiently for the opportunity and then took it. No hesitation or mercy.