I could write a longer post about how crucial the storytelling is for me, but I'm on mobile at the moment.
For now, I'd say it's best to have some motivation behind all the killing. Context is needed to create an atmosphere, to provoke a specific mood on the player, to help you believe the world and the situation you're facing; the illusion of a reality. And, if for a moment, the player gets immersed in your narrative and forgets about the outside world, then you haven't built just a game; you have created a work of art.
Of course, all of this would be useless if it isn't backed up by a robust and fun gameplay. Both story and mechanics need each other to reach this "work-of-art" status.
As a final thought, we wouldn't need a fine studio such as BUNGiE if the game was only about shooting aliens, aren't we?