It is not your decision to kill another person who is suffering.
Suicide is against the law in most of the civilized world. The law is gentle when the suicide attempt fails and help is usually given.
The problem occurs when they can't make it, as we have seen in cases recently. Just to create another dissonance for you. If I am the decision maker delegated in an advanced medical directive and I'm asked whether or not to "pull the plug" on another person, who's decision is it to do so?