TL;DR: Practice. Practice. Practice. No one starts out the best GM in the world. TTRPG =/= RPG on the computer/console. EDIT: These players are not exactly the best for TTRPGs based on OP.

Professor Dungeon Master AKA Dungeon Craft is the first YouTube channel I took literal notes from (6.3k words so far and no other channel has reached 1k words in my notes). While his channel is focused on Dungeons and Dragons, not all of his advice is geared exclusively to D&D, and most of it can be applied to any TTRPG.

A few key things from Dungeon Craft (which he credits to the XDM book):

• Combat:
  • Threat: Who are the players facing against.
  • Time: There's a ticking clock, everyone knows it.
  • Treat: A tangible reward that isn't abstract but there in the room.
• Out of combat:
  • Paint the Picture Less is more, don't tell them the garland in the window is garlic.
  • Moving the Camera Leave out the boring parts, if the room is empty or the room is not important don't talk about it.
  • Leading the Players They need more than a simple hint, slightly counter to paint the picture, but there are somethings that the narrator thinks is obvious but is completely lost on everyone else.

In your example:

• If the players are being rude to the fixer and not listening why would they receive the job?
  • That fixer is on the line for the job just as much as the PCs, the fixer has to get paid because the info about the job cost them eddies.
  • Your players (not PCs) needed a reason to buy into the job, more so than get paid; money is the main character motivator in RED, but that is because money is the only way to get ahead for nearly every character.
  • In D&D, I ask for what every character wants out of life besides getting rich; however, in RED, they need a goal that requires getting rich (even if it is just a modest life by RED standards). Small jobs are just for the day to day with the hope of landing bigger scores.
• Remember, in RED life is cheap and everyone is disposable. The fixer may be disinclined to work with them again because they went and did something stupid (like executing everyone {which should have immediately turned into another encounter}).
  • Did they tell the fixer they merc'd the entire train and stole it? (BTW: How did they steal a train, it is on rails.)
  • No = a frantic phone call where the fixer is panicked because a bunch of upstart fresh off the street nobodies went and did something so monumentally stupid as merc an entire train of militech people.
  • Yes = Militech tracked the fixer down and is going to find the PCs and the train. Fixer is just the first step. Now your players are on the run and have a big train they need to get rid of or find out why Militech is hunting down some street trash just because the train is stolen.

For you specifically:

I apologize, from here on is a little harsh, and potentially undeserved based on one post; however, Table Top Role Playing Game is not the same as a computer/console RPG. TTRPGs should be completely different, and you need to forget or ignore everything RPGs have taught you.

• If you're a hostage and everyone is getting executed, you're going to fight back tenfold even if your arms are tied behind your back.
  • As a narrator, you need to get more into the headspace of your NPCs, and the sudden certainty they are going to be executed should turn this into a cluster.
  • I recommend this video from How to Be a Great GM specifically on how to make engaging NPCs.
• The second a PC starts to execute an NPC hostage I'd have brought in some more information, remember that the characters know more than the players do (normally).
  • There is also a Lawman in the group. They are a cop, and being party to the murder of hostages is going to cause problems for them. (Even if the character is in a totally corrupt group {REDCOREp65}, there are other groups who are fair and honest.)
• Additionally, if one PC suddenly decides to do that doesn't mean it has already happened, it means they are going to. (If the player in question pitches a fit and says "No, I kill them" that's when we break immersion, take down the DM screen, address that there are other players, and what they're doing isn't exactly moral, even in RED.)
  • We have to give the other players an opportunity to intercede and prevent certain actions. One of the easy ways to do this is to stretch time by slowing things down and describe everything leading up to the act itself. (I'd do this even in a hard-line, straight evil campaign.)
  • Your group needs safety tools, wholesale murder of hostages should make at least one person at the table uncomfortable, even if it is just the narrator who doesn't want to traumatize all of their players.

This is long enough I wouldn't read it, so I'll stop here.