The Scab Sniper in Darktide is a highly dangerous special enemy. If he's caught close to your party then he will sprint away to a distant position. From there, he will aim sniper shots at you. You can see where he is aiming from the laser sight, giving you a short window to dodge the shot. If he lands a hit on you, you will take massive damage - on higher difficulties, one or two of these shots can be enough to take down a player.
If someone on your team is able to snipe distant enemies, they should kill the sniper as soon as possible, otherwise you should utilize cover and alternative routes where possible to close the gap to the sniper.