IMHO an entity is ethically responsible for anything its contractor does, unless there is reasonable clarity that the contractor acted in opposition to the client's wishes. Having someone else do your dirty work doesn't make it less dirty.
It's a pretty rotten analogy.
Randstad hired the people, Google contracted Randstad to do a job. Randstad's people did it in a shitty way that is hard to expected.
It's a pretty fresh analogy.
No, you are not responsible because the teen was not operating within the terms of the contract. You did not authorize or ask him to get stoned or run someone over.