Just then, they see one of the Nilfgaardians survived, lying nearby. Roche wants to spare him while Ves states they need to kill him. Ultimately it comes down to Geralt's decision, so choose whichever path you wish. If you choose to kill him, Roche will finish him off despite being against it.

What happens if you kill the Nilfgaardian?

They aren't meant to be killed. In the tutorial area they are there to be designed to teach you not to steal from people while guards are around. If they kill you you just wake up after a while and lose some money, you don't even have to reload a save. If you kill them they simply "respawn" and go about their day.

Should you save the Nilfgaardian soldier?

The quest will fail if the player does not save Ves in time. There will be 7 Nilfgaardian soldiers in the way to Ves and 5 more that is fighting Ves. Choose to kill or spare the surviving soldier. It will not have any gameplay nor story impact.