The game gives you the option to kill the Shaelmaar.
Normally that kind of option would influence the outcome of the story somehow.
In the Shaelmaar's case, does it influence the story? Do you get any reward whether you kill him or not?
If you spare the Shaelmaar you will get one of five virtues.
The virtue that you get is Compassion.
There is one side-quest ('There can be only one' Quest) in which you must have all the virtues. (I won't spoil anything).
So all-in-all, you sparing the monster won't have any immediate effects or any large repercussions, since Compassion is one of the virtues you can get in another situation in the DLC.
Just FYI and to feed curiosity the virtues are: