For reasons that would take to long to explain here, my campaign requires are certain feel of fear and paranoia for my PCs. There will be quite a few failed assassination attempts and so on. I'm running The Dark Eye, which is an extremely realistic game.

To really deliver this feeling of threat and fear for the lives of character, I want to kill an NPC in a fashion that the death is most dramatic and touching.

The NPC is a member of the watch and will only be around for somewhat 3 to 4 hours (3 days in the game) [1]. He will save the lives of my PCs 2 times.

Are there any other measures and improvements I could take to make the death as dramatic and with as much impact as possible?

[1]: 2 days is not much in "real-life" to get attached to someone, but I assume that if there was a policeman or policewoman protecting you over 2 days and they suddenly get slaughtered, you would be quite scared.
In a role-playing game, you have one more layer or abstraction and players will usually not relate to the fictional world as much. This is the main difficulty that I can envision.