If a character has Sacrament (probably also works with any other sources of elemental damage increase) and then becomes poisoned and kills the source of the poison, the character becomes the source of the poison and deals increased damage to themselves.
Test this by buying sacrament from a pardoner, taking a low level character to Tenet Church B2, let an Infro Holder Archer poison you, then kill the Infro Holder Archer. The poison will now do bonus damage to you based on your bonus elemental damage. In 9 out of 10 cases the damage is enough to kill the character.