[Manuel_D]

No, it's entirely viable to say a group of people did something. E.g. "a lynch mob killed ______." Sure only one member of the mob actually tied the noose, but the mob as a collective took that action.

[thunderfork]

"one can say a group took an action in some cases, so it must be right to say a group took an action in this case" is not compelling.

[Manuel_D]

The point is they're both cases where a group of people get together to carry out a criminal act of violence. The fact that only one individual actually fired the gun or tied the noose doesn't alter the fact that the group as a whole carried out the criminal plan.