Ever since WWII, no nukes have been used in open conflict and there has been a serious effort to limit their spread. As for biological weapons, they have not been used by any major power since WWI and anyone who did would become an international pariah (e.g., Saddam Hussein after gassing the Kurds). The Engineers appear to have no such scruples and bio-engineered monsters from one of the most dangerous life forms in the universe that have the potential to render whole planets dead. Also, it's again clear from their artwork that they were quite proud of their creations. The equivalent would be if the US had a dirty bomb factory right in the middle of Michigan with a giant statue of George Washington on top holding a cross in one hand and a bomb in the other, with murals on the inside of the factory elaborately detailing the deaths of people who had been attacked with the bio-weapons. Outside of terrorist organizations and North Korea, no government today revels in death and destruction to the extent that the Engineers did. And even if judging the Engineers from what we saw is unfair, the second it was clear that dangerous biological weapons were present, the responsible thing to do would have been to take off and radio in the authorities, not to act like they were in the Land of Oz and that the Engineer was the Wizard.