Similar to the first two Metro games, Exodus has a hidden karma system. Unlike, say, Mass Effect and its transparent renegade/paragon system, it's not always clear what constitutes a 'bad' action in this game. But it mostly boils down to common sense. If someone asks you to retrieve a precious family photo or a teddy bear, it's probably gonna net you some good karma.
But you'll get on the universe's bad side by killing enemies who have surrendered. Kill enough of a bandit's allies and they'll drop their gun and put their hands up. You can still execute them, but unless you're role-playing Artyom as a senseless butcher, you probably shouldn't. It's bad for your soul.
Conversely, talking to people with your weapon holstered will earn you good karma. To holster your weapon, hold down the H key. Some characters have lines of dialogue recorded especially for a player who approaches with their gun lowered. If something feels like it's the right thing to do, it probably is. But your mileage here may very depending on your own moral compass.
If you accrue too much negative karma, you're gonna get the bad ending. And while I think a 'bad' ending is as valid as any other in a game, it's thoroughly depressing, and a deeply unsatisfying way to end the Metro trilogy. So make sure you behave yourself as you explore the wasteland, and go out of your way to help people, otherwise you're gonna have a bad time.
Ultimately, Metro Exodus is kind of a role-playing game, so do whatever you like. If you want to be a total shithead, that's up to you. But just know that you might spend the end credits staring listlessly into the middle distance, wondering why you did the things you did. And honestly, after all the shit he's gone through in this series, poor old Artyom deserves better.
Do it for him. Please.