It's not always as easy and as simple as that. To assist in a loved one's suicide is not something that can be done on a whim - it affects everyone. Think about it, not everyone wants to or can hold that much responsibility for someone's death - a loved one no less.
And I've said in previous posts, the situation is this:
A) Letting them live in pain for a little while longer and then die naturally
B) Pulling the plug (so to speak), so they die now.
So, the decision for taking away life is in that situation not easy. You aren't holding a gun to the person who has raped your wife and slaughtered your children. You are ''pulling the plug'' on your wife or children.
The decision either way is not always simple.