You could always look at movies and TV shows to see how their do it. For example, I heard Bojack Horseman does a good job of this (never seen it, but it has been praised for that aspect).
And also Rick and Morty? Maybe a little Steven Universe?
I think it would depend on the overall tone of the story. Is it mostly dramatic or mostly comedic? If it's one, downplay the other just enough that it still works as a dramatic/comedic scene but it's not SO dramatic/comedic that it jars the audience when you flip from one to the other.
Well, first you gotta ask yourself "why would the main character execute the leaders even though the leaders agreed to disband their gangs?"
You can use a character as a vessel for the audience, so they are eased into the drama. Maybe have one character still smiling (as you expect the audience to do after a funny scene) and let the smile drip from his face while he realizes that something is off. You can shift the tone slowly by showing slightly darker things, letting the character realize that he has to be on guard or something.
You can do the same fairly quickly, if you feel comfortable with it. That can have a shocking effect on the reader as he realizes that the scene is not as peaceful as it seems.
Just my silly thoughts. Best of luck 
Alright, cool. You can use this moment to visually tell the audience exactly that. "This is how the future is: No trials, no mercy."
If it were up to me, I would make the transition from the lighthearted moment to the executions abrupt, just so the audience would get the message about the world in your story. To make this work, though, you need to have the tension raise. Build that suspense, segway into a lighthearted moment, and snap back to that climactic execution.
So its a rough idea and the goal is to "build up the tension" but one way I can see it being executed would be:
the cops are carrying out the interrogation, it's a calm conversation. maybe a joke has been cracked, sarcastically, nonchalant however. Like you said the leaders of these rival gangs admit they've forgotten the reason why they started fighting. maybe laugh at the stupidity of it even. then the cop interjects either clearing his throat or his laugh just seems off. he either makes a statement about what the laws are, or mentions "how things work" or maybe says nothing at all and just approaches these guys drawing his weapon. maybe there's a look about his face, in his eyes, the way he's moving and how these guys are reacting to how he's creeping up on em. there's that feeling of unease and then bang.
i always like to look at how a tense or suspenseful movie scene is played out. remember how they managed to create that sense of unease. was it something that was said. was it the way a character said it. or was it more sensory. body language. how something looked. a smell in the air. for me if something can play into that and give a good sense of dread or unease, even if it's done in a brief moment, then that's what'll really get me. i ah- hope that helps
You could just totally not transition... So you have the main character and gang leaders negotiating, they come to an agreement, it's light hearted...if it were me, I'd even make it a little heart warming. You know, like they get each other, not sure why they were fighting, but it's important they stop all the senseless killing and just go back to being one big family. As soon as this is resolved and everyone is on the same page, the main character shoots both of them. There isn't time for either of them to react, its just over. In this scenario, its the surprise in and of itself that changes the tone of the scene. I wouldn't try to build tension or hint that this is about to happen. Killing them without warning, is going to be dark enough.
I do have a question though. Why spend the time brokering a peace treaty at all if they're just going to be killed? Would the rival gangs stick to the deal if their leaders were executed directly after in cold blood? I don't know the story, but it almost feels like some sort of sadistic joke on the part of the MC..
I also don't get why the MC would take the time to negotiate peace if they're gonna kill them anyway. Why not just kill them and be done with it? What's keeping the gangs from continuing the fight now that their leaders, who agreed to call the fight off, are now dead? If anything it would create a power vacuum that the other gang members would be eager to fill which would not only cause the gangs to keep fighting but fight each other for the positions.
Well, I was planning to make the execution scene inferred to be after the gang leaders disbanded their gangs and a bit more humane than just sudden gunfire and death to show that the MC isn't heartless.
The original mission was to simply track down and kill everyone in both gangs, but instead, after the gang leaders disband their gangs, he opts to only kill the dangerous ones and place the ones with weaker abilities on probation in hopes that the ones with weaker abilities wouldn't have the strength to startup another gang.
The post above probably answers most of your questions as well, so the leaders obviously would have reasoned with the other gang members to simply stop fighting and disband the gang.
A plot I'm toying with is that the gang members that were spared decide to revolt against the government or make the MC regret sparing them in a revenge arc or something though.
Well, I have to agree with @MichelleAntisocial's version of events.
A sudden execution, Puella Madoka style, should do it.
But, if you want to put some heart into the MC, then maybe you could fatally wound them. Give them a slow wound, and then let the MC show remorse, and the gang leaders could be in disbelief, then surprise, then finally, peace.
That's how I see it.
Well, the scene isn't supposed to be shocking and contrast between the general lightheartedness.
It's supposed to be dramatic and show the darker sides of the world but also that the MC isn't heartless. He has to carry out his job, but is willing to grant the gang leaders a quick painless death.
Ah. Okay...
Maybe you could put it like this:
Two gang leaders start talking about disbanding their gangs. MC jumps in, starts talking with them. (Add some jokes and philosophical stuff here.)
While it's steadily getting darker, make the MC show regret "for something". Then, when the gang leaders ask why, MC executes them.
Then you could give him a soliloquy, Macbeth style.
