Merkel wants to have stability in Germany so that her government can try to restore the economy. This is her way of appeasing Iran in hopes that Iran doesn't cause any trouble for Germany. She also forced Mehlis to resign from the Harriri investigation so that Germany could improve its relations with Syria.
(Same policy followed by other countries, like Saudi Arabia, France, UK - appease the Islamists in hopes they'll do their dirty work somewhere else.)
I can't think of a better way of killing someone.
The US says please don't release him. Germany says we are going to release him. Man disappears. Man still missing. US says we are going to find him. Perhaps all along they released him to US and we killed him.