no, it can be for other reasons, for example, self defence, a disagreement, to gain territory, for profit, expression of rage, etc etc. NB :these reasons might not be right or wrong, but they are motivations outside of seeing someone die.

The idea is that in "I shot a man in Reno, just to watch him die” there is no other motivation. The only motive is to see the guy die, in a singularly morbid motivation.