Millburn is a biologist. He's interested in lifeforms but not dead alien lifeforms, and the creatures seemed harmless to him. His discovering a new alien species probably outweighed his judgement or his fear, which is completely nonsensical that he would be intrigued by a living specimen, but scared and alarmed at a heap of dead Engineers. He might have expected the curious-looking creature to hold still or, at worst, run off, since at that point he had no real reason to believe it was hostile. The only real indication of its nature is its resemblance to a King Cobra, which for a biologist would not be taken as any sign- until such time that it also acted like a King Cobra, and any biologist worth their pay packet, would act like a biologist and not attempt to pet it. Chances are, if a creature whether or not it be alien or not displays characteristics of a creature you are familiar with, and know those characteristics to be threatening and dangerous, you just DON'T PET IT! Some claim that it vaguely resembles the facehuggers from the other Alien films, this would be people that are stupid in nature, as it looks nothing like a facehugger, now or ever! He might also have figured that his spacesuit would provide adequate protection against any possible dangers posed by the creature, but most sensible people just wouldn't take the chance, after all, it is an alien creature, acting in a threatening manner with unknown strength, and also there is the chance it might be highly toxic, which it proved to be!