Killer and Immortal Target

How do you kill a target that can't die? Even worse, how can you possibly kill said target if they give you food, clothes and a hot shower? Will Koroshiya-chan be able to kill the mafia boss Leonard Amaretto? On top of that... will she be willing to, even when the old man asks her to do so after receiving the kindness he shows her? *Note: Chapter count includes 2 extras.*