Why did Thragg leave Mark alive?

Other than plot armor, is there a reason Thragg didn't kill Mark?

My immediate thought, based on what he said when he spared him, was that the fact he had viltrum DNA was useful. I can only conjecture it is for breeding purposes (trying to get maximum bio diversity), assuming he believed he killed Nolan already.

Then Thragg gets to earth and spares Mark again for his permission to infiltrate the planet??

He could have just killed Mark and then went to earth and infiltrated.