Before the "deathers’" lies meet their own timely end, let’s look at who invented them.
The mandatory government death panels were invented by free-marketers and promoted by true-believer conservatives. Their reasoning is that pure economic, utilitarian considerations would determine whether Grandma lives.
It is a classic case of projection or, in political terms, attacking your opponent with your own weakness. Conservatives have invented a mechanism that aligns with their amoral, market-driven thinking. That is why it gained such currency on the right: it made sense to them. That is why it was so hard for the left to challenge: it was simply so nonsensical.
Also note that, in everyone's favorite red herring, the British NHS, prescription medications are free for those economically worthless seniors and children. So much for that argument against socialized medicine.
When their argument makes no sense, look for a a projection of their own mindset.
UPDATE: links added as requested