Why did Frankie Marino kill Nicky Santoro and his brother with such confidence

The mob bosses had sent Nicky to Vegas to basically represent them and control the situation in Vegas. Nicky, at first had kept the bosses satisfied, but eventually he got completely out of hand and had transformed himself into the new "Godfather of Vegas". He got so out of control that the bosses couldn't handle him and Nicky just had to go. Thats how the Mafia works, you go a little out of hand and you're out.

Now, everyone in the Mafia, The Bosses, The Capos, The Soldiers, and of course the associates too have to follow the Omerta. Following is a list of possible reasons why Frankie killed Nicky and his brother Dominic so ruthlessly:

1. He was just following orders and did what had to be done. In short, just fulfilling the necessities and following the bosses orders,

2. Secondly, assuming he had a "personal problem" with Nicky and he knew that Nicky had it coming in the end, Therefore he held it all in and took his "revenge" by simply following the bosses orders.

Now, even if he did not have any problem with Nicky, you must have noticed that Nicky had an "understanding attitude" towards Frankie when he was beating out Dominic in front of him. He understood it was all the bosses's orders and he had to go out along with his brother and of course being made and example of.

So at first he was furious but eventually he understood and let it all go. Frankie, on the other hand, If he did not follow the orders he'd be in danger himself, Hence he just did what had to be done

Nicky had forced Frankie to lie about the affair and at the end Nicky tries to kill Sam and Frankie cared about both of them so he saw Nicky's multiple betrayals and saw Nicky was gonna keep coming back to betray so it was a major F*** you for making me do this type stuff.

I think they tolerated Nicky despite his anger issues until he started sleeping with Ace's wife which i believe was the last straw which broke the camel's back, Frankie had to do that to make up for his mistake of covering up that affair as well and besides Nicky started bringing in a lot of heat by killing cops and breaking all the rules.

The mob bosses had allowed Nicky to run rampant out in Vegas, but they eventually had enough of his antics. The beating was a message, that's why Nicky's brother got beaten to a pulp first. If you cross the mob, or don't play by your rules, you'll live just long enough to see your loved ones killed.

IIRC, by carrying out that hit on Nicky and Dominic, Frankie and his crew were given "clemency" for not reporting Nicky's affair.

Towards the end Nicky was out of control. The bosses were fed up and also understand that Frankie had done some jail time as well. That would be cause to have some deep anger towards Nicky. And of coarse he had to follow orders. His loyalty to the bosses was obviously more important. And understandably so, they were his lifeline

Frankie was between two fires: the bosses and Nicky. He covered Nicky's arse as much as he could. Nicky though has clearly shown to Frankie that he can betray (Ace, the bosses) as he didn't care about anybody. So Frankie wouldn't be an exception, say, if bosses learn that Nicky slept with Ginger. Frankie would be wacked for that and Santoro wouldn't care. Besides, by wacking Santoro Frankie made sure that the bosses won't even learn that he lied to them. So he stays clean.

Ace was not directly involved in the skim. He merely brought the money to the casino. Note that bosses wasted only those who had direct access to the counting room i.e. were directly involved into the skim: Andy Stone, Nance, the counting guy and two others. I believe all of the white shirt counters might be killed too.