Maybe, if the regular teacher is ok with the idea, you should invite a biologist who likes herps to give a talk to the class about the importance of the food chain, pest control, etc. If you give the talk yourself, they won't pay attention because you are just another student. But if one of the science teachers did it, or somebody from the local nature center, etc, then maybe it would make them think, at least for a minute or two.
I also get angry and disgusted by remarks or experiences such as you describe. But I learned a long time ago that showing your anger doesn't really change minds or behavior. Trying to find a solution, even if it is only a small, "drop in the bucket" kind of solution, seems like a better use of time and energy.
I also have employed a technique that sometimes works in certain situations because it appeals to greed. Example: "I just killed a 10 foot rattlesnake yesterday!" Reply: "That's really a shame. One that big is really, really rare. Might have been worth a few thousand dollars. Too bad you threw that money away." It may or may not make them think a couple of minutes next time. But at least I feel better that they might think they lost a lot of money through their stupidity.
Another example that may or may not help: "I just killed one of those rat snakes yesterday - I hate snakes!" Reply: That's really a shame. Rat snakes eat the same food as rattlesnakes (copperheads, whatever). Now there will be more food for the rattlers in the neighborhood, so you might start seeing more of them."
You can make up some good replies of your own for various stupid remarks like that. Have them ready ahead of time. For maximum effect, you have to give the reply without apparent anger, and a touch of sadness that the stupid person will have to suffer the consequences of his act.