It is fruitless because it is a pointless argument.
Of course people are behind the deaths, but saying that "Grenades don't kill people, people do" is hardly an argument to allow people to carry grenades (Although road rage would be a bit more exciting!
).
It is really a question of "how easy do you want it to be for those few people that want to kill as many people as they can, to do so?"
Note: I could also argue that your point is mistaken, because in the case of a toddler that shoots someone (and is too young to really understand the idea of death or recognize the real gun is not a toy), I would argue that it is the presence of the gun that causes the death, not the child.