Assisted suicide: where do nurses draw the line?

If a terminally or chronically ill patient chooses death, where does one draw the line between allowing suicide, encouraging suicide, assisting with suicide, or directly killing? How does one differentiate between injecting a consenting patient with a lethal injection for the purpose of causing death, from inserting the needle so that the patient may inject the lethal dose his or herself? Clearly there are differences between action, facilitation, encouragement, and mere information.