As a neverending war between humans goes on, a growing concern for environment becomes apparent. I propose terrorists use environmentally conscious warfare products/tools to kill humans, this way they will achieve their goal but will still retain the beauty of environment in its pure form.
Lead-free bullets is one of such solutions...
http://www.cesaroni.net/ammunition.php

Any other suggestions?

Tungsten is a possible alternative to lead, but expensive and difficult to process.

You cannot poison the enviroment with a naturally occuring metal that isn't radioactive.