Suppose your love one is very sick. His/her illness is already terminal and he/she is deemed to be hopeless.
Would you help him/her to end his/her misery without even asked by him/her personally ?
Of course, this would make you a killer literally, but I believe "some people" can understand what you've done.
Personally, the more I love the person, the more possible I'll do it for him/her. My reasons ? simply I don't want to see him/her living miserably any longer.