I’m fairly certain that the extinction of the various other hominid groups followed a similar pattern that includes initial contact, the spread of disease, and then followed by warfare and conflict, absorption and subjugation.
Without a doubt some human groups have knowingly used germs or bacteria in warfare. The Mongols, Genghis Khan, used every possible known tool or strategy, including lobbing plague ridden corpses into cities under siege, to conquer more of the known world than any other conqueror of known history. However, Khan and his hordes had no way of knowing how susceptible the people they were fighting were to plague, and of course such methods would often backfire.
No human group or invader could have possibly known that they were spreading diseases that could or would help lead to the extinction of other hominid groups they came into contact with. Similarly, the Spanish, French, English, and Portuguese, had no way of knowing the spread small pox, and various other diseases, to Native American populations would have such a devastating affect. I’ve heard some people use the word, “genocide,” to describe these types of incidents, and that is a mistake.
Genocide is the intentional effort to exterminate a group of people with premeditation. It is first degree murder on a large scale and usually based on killing off an ethnicity, a culture, a tribe, or a perceived rival group. We call smaller scale efforts such as this massacres. While the brutality of warfare, and germ warfare, used in antiquity or during the modern era, is not excusable or acceptable. They would be described as massacres, not genocide. The actors in these instances had no foreknowledge of the susceptibility of those they were infecting. They could not have had premeditation of the scale of the damage they might have caused. The conquistadors did not know how susceptible the various Native American groups would be to Old World (Eurasian and African) diseases. They may have knowingly spread diseased blankets with small pox, just as the Mongols spread various plagues in their conquests, but neither side had any way of knowing how devastating diseases can be to immunodeficient groups.
We hope the modern trappings of civilization and institutions such as the the UN and Geneva Conventions will help contain the harsher nature of humanity, but we all know that violence continues around the world on a large scale. It’s almost like parts of the world are now in perpetual conflict and chaos as the fabric of societies and environment degrade.