Full Idea
Utilitarians have two direct objections to killing (the fear and pain, and the loss of future happiness), and two concerns about side-effects (the loss to friends and community, and the bad precedent and public anxiety caused).
Gist of Idea
Utilitarians object to killing directly (pain, and lost happiness), and to side-effects (loss to others, and precedents)
Source
Jonathan Glover (Causing Death and Saving Lives [1977], §4.4)
Book Reference
Glover,Jonathan: 'Causing Death and Saving Lives' [Penguin 1982], p.71
A Reaction
This invites obvious counterexamples, of somewhat unhappy, lonely people, who can be quietly killed with no qualms. Who will be callous enough to do this deed for us?