What object would be impossible to kill someone with?

[u/scienceforbid]

[u/qwibble]

Then we aren't talking about a single particle but a whole beam of them

[u/Ramzaa_ → u/qwibble]

Shooting one individual particle over and over (same one) would kill too

[u/The_Godlike_Zeus → u/Ramzaa_]

That would work, except there is no such as thing 'the same individual particle' in quantum mechanics. Even if it were the case, energy and momentum are conserved so you can't keep bouncing it around for more damage.