Tallister

 was a Conglomerate scientist sent to vivisect a Laan. He was the number one authority on xenobiology. He wouldn't complete the vivisection has he didn't want to end up killing the creature. He killed the Laan he was dissecting. He helped the Doctor to find a way to convince the Laan that the Earth wasn't their birthing ground. (AUDIO: War Against the Laan)