1) Natural, yes. Humans kill each other everyday. But, unlike other primates, our minds have conceptualized that "us" extends beyond our local group and recognize that all of us humans are in this together, and that we don't need to kill each other to fight for land or resources (though many of us still struggle to understand that).
2) Mantises aren't social animals in the way that humans are. An individual mantis can go out and live its life isolated from its kind and be just fine. The only contact it requires with another mantis is in reproduction. You would do better to draw a parallel from an animal that lives in groups (like apes or elephants).
3) I believe lots of primates, cetaceans, and canines regularly engage in homosexual contact, to name a few. Are you that hung up on anal contact? Because straight people do that, too. And if you're so rigid about sexual contact being for reproductive purposes only, I hope you're just as concerned about oral sex.
You can read some examples (including anal sex!) here:
http://en.wikipedia.org/wiki/Homosexual_behavior_in_animals4) Why is your bible the basis for any law? If the country were 51% Hindu, would we be talking about banning the slaughter of cattle for food?