Normally, we don't answer homework questions, but your assignment actually seems to mandate such help, so I'll bend the rules in this case.
1. This is certainly the hardest and most subjective of the three. It is a matter of debate whether these responses reflect genetically based predispositions or cultural traditions. Some cultures ignore or even revere snakes. However, there is good evidence that many nonhuman primates have an innate, instinctive ophidiophobia and that humans in Western societies acquire it much more readily, from early experience and the behavior of adults, than fear of other objects that pose much greater threats, suggesting a retained, adaptive, negative response to elongate creatures. For example, I adore snakes and kept many as a child, but I remember once being surprised by a small garter snake on a trail and having a severe, visceral flight response. So, in summary, it seems that humans may be genetically predisposed to distrust snakes, which our subconscious psyches view as potential danger, but that this response can be overcome by cultural conditioning.
2. Absolutely, on the whole, humans pose a much greater threat to snakes than vice versa. The number of snakes injured and killed by humans - and their cars and pets - is infinitely greater than the other way around (the same is true of all other "dangerous" animals). Furthermore, humans threaten not only individual snakes but entire snake species due to overhunting, prey depletion, invasive species, and habitat destruction. Humanity, on the other hand, is in no danger of being wiped out by snakes.
3. This is a long list. Benefits to humans include study subjects for biology; products such as food and leather; pest control; and pharmacological and biochemical uses of their venom to prevent heart attacks, strokes, and cancer. Environmental benefits include serving as food for other animals and homes for their parasites, and keeping prey populations in check.
Ken Burton for eNature Naturalists