Tough topic. I think people should have the right to make that decision for themselves.

I think it's okay to "assist" if that assistance is to help bring the person and means together... but...

The person committing the suicide has to be the one, and the only one, with the "finger on the trigger". S/he has to swallow the pills, or close the circuit, or inject the drug, or tie off the plastic bag... or whatever the means of suicide, themself.

Smothering someone with a pillow, for example, takes away the possibility that the person committing suicide could change their mind at the last moment.

It's only characterized as an act of mercy (at this time) because we don't allow suicides. Give people the right to end their own lives and an act of mercy is no longer necessary.

So yes, they're murderers imo, but whether or not they require punishment, goes to the question of gain (as you pointed out.) But I would much prefer to simplify the question by making such acts unnecessary, in which case, such acts would clearly be wrong.