When attempting to write believable interactions between characters in your story, one of the most important things to keep in mind is consequence. Your characters are going to make choices and decisions based on the information they have about a given situation, and those choices have got to have consequences. No matter what the setting, creating a believable reality is still paramount to capturing the reader's imagination. Whether or not your story has magical elements or flimsy morals when it comes to the laws of science, this law must not be broken.
For example, say character X is put in a sticky situation that requires an immediate reaction. He's in a shady part of town where the local police force is rather heavy-handed, and he gets cornered for a shakedown. Our hero is currently in a jumpy mood, and so makes a choice to try and escape rather than submit to this injustice. So he makes a move for the officer's weapon, wrestles it from him, and accidentally knocks the other a little too hard against the wall, cracking his skull. The other officer, now on the wrong side of the gun, pleads with your character for mercy, to which the protagonist responds by gunning him down.
THIS. SHOULD. MATTER.
Have you ever watched a tv show, or read a book, or engaged with some other story in which the protagonist makes all kinds of wild choices, without ever reaping the consequences of their actions? It's not inherently a bad thing, but there is a limit to what is called "The Suspension of Disbelief". In short, the reader should be able to understand where the limits lie in your story as to what the characters can and can't do without drastic consequences. In the above example, our hero has just made a grave mistake by critically injuring an officer to the point of death, and then making the conscious decision to murder the other one so as to leave no witnesses. You then as the author now have a choice of your own to make moving forward. Will this character's actions warrant repercussions? Or will he get away with it scot-free?
It can be tempting to let your characters get off without so much as a slap on the wrist, but trust me, your readers will find your story much more engaging if they feel that your characters are capable of making good and bad decisions. And the consequences do not always have to be so binary. Bad choices can be rewarded, and good decisions can be punished. It all depends on how you want to present your narrative and world.
If I were writing this story, I would think of this scenario as a good plot hook to get my story started. And no, I did not just steal it from a popular tv show that is currently coming out, I have no idea what you're talking about.