The ending is unreasonable (spoilers)

Because he knows that Howard will just dick around again and scheme up a way to not pay them. That is kind of the point with Howard - he had several opportunities throughout to break even and just go home, but to some, there is never enough. So, he decided he'd rather handle it the way he did than to just continue to repeat the same process over and over, and get the money they were owed (and probably more) in jewelry.

BIG SPOILERS (of course)

I disagree. Arno was Phil (killer) and Nico (chubby henchman's) boss.

1. Phil and Nico, the two thugs, weren't on their own, they worked for Arno. Whatever their desires, they had to clear it with Arno. Since Howard owed Arno money, robbing the store would be taking away inventory which had to be sold to make the money Howie owned Arno. Plus, the reveal at Pesach that the two were brothers explains why Arno didn't have Howard seriously injured or killed long before this. Howie was the "preferred" younger brother. (I know that feeling).

2. Financially speaking, most of the inventory is either "floored" meaning owned by a lending company (or, knowing Howard, loan sharks). Maybe the dad even owns it all. Thus stealing it means fencing it for less than it could be sold for, with fewer proceeds going to Arno.

3. The thugs are on Arno's payroll. Phil kills Adam in an explosion of anger and pent-up frustration, probably from years of suffering his BS. He doesn't do it on Arno's orders. He knows he'll be in trouble (at best) with Arno. So, he kills him too. Now they have to cover up the crime so they stage a burglary. THAT's why they trash the place, not to make a score. If they fence the gems, they'll be discovered and probably be on a hit list of others of Adam's creditors, so they'll probably dump 'em all & skip town.

4. Phil and Nico aren't mental giants. There are cameras all over the store & at the end they are about to leave, with no thought to stealing or erasing any video. So, I figure they're doomed.

5. The father, played by Judd Hirsch, is probably a kingpin who staked both Arno and Howard. That would be another reason why Arno put up with so much over the years. I hope he loses his money. What an A-hole! Plus I didn't like his character on Taxi, either. :D

Just came here to say that because Arno and Howard are family, that's why he never actually has him killed, but fitting in the theme of the movie he got killed because he finally run out of second chances.

The implication was that he was a hothead, and someone who could not stand to be at another's mercy, so after being locked in that cage for 2.5 hours or so he was well beyond thinking straight. He'd probably sat in there watching Howard gloat, feeling powerless, and telling himself that he's going to kill him as soon as he's free. In a blind rage, he did just that.

1. We're not sure how much of that 1.6 million Howard owes to his boss, not even HIM. He's just muscle. Who knows how much of that he would get.

2. He stole millions and millions of jewelry from the store which, even if he is able to sell for only a fraction of the value, will be much more money.

3. He was fucking pissed off at Howard.

4. He's a violent fucking criminal and might not always make the best choices in life.

The BIGGEST reason that this ending was strong: the surprise factor.

After watching these guys put up with Howard's BS for two hours and even take his abuse (I believe that he slaps Phil at one point) only to let it slide, you begin to realize that it's not personal with these guys. They just work moment to moment, situation to situation, doing what they need to to get their boss paid, until the boss waves them off. It's all transactional. Even after getting fucked with to the point of being locked in a tiny room for two hours, you think that because they're about to get paid, all is 'forgiven'.

Then you realize that a combination of opportunity and rage tips the scales just when you thought that you knew what the rules were between these guys.

Sometimes when people get upset, they make poor decisions.
Phil was demeaned and took it personally, saw red, and wanted his revenge.

Sandler’s character should have told the guy to drop the gun/or get rid of the bullets.