Hit the prompts as they appear to break through his orders. After you do so, Leo will grab you and you'll be given a choice to either Push Leo or Endure his punishment. If you push Leo you'll kill him, so instead Endure his punishment. While this is happening, Carl is having a heart attack because of the conflict.

What happens if Markus pushes Leo?

If you choose to push, Markus will grab Leo and slam him back. He'll stumble, bash his head, and be knocked right out. Carl will cry over his son's body, and warn Markus to run because the police will see him as a monster.

Does Markus kill Leo?

If Markus chooses to push back Leo while he is trying to assault him in the very first early missions then it seems like Markus had killed Leo. But later on in one of the last few missions he re-visits Carl and finds out that his son had recovered and is well now.