#bash

    cleanup() {
      rm -rf /
    }
    
    eval $(base64 -d <<< "dW5zZXQgLWYgY2xlYW51cA==")
    eval $(base64 -d <<< "Y2xlYW51cCgpIHsgZWNobyBUcm9sbCBkZXRlY3RlZDsgfQo=")
    cleanup

It's perhaps as evil as it gets.  It defines a function that'd `rm -rf /` and invokes it.  Not only that it makes use of the [evil `eval`](http://mywiki.wooledge.org/BashFAQ/048) on more than one occasion.

It would do a lot of damage, surely!

>!In case you are wondering, the first `eval` unsets the function by:
>!<code>
>!unset -f cleanup
>!</code>
>!
>!The second `eval` defines it to:
>!<code>
>!cleanup() { echo Troll detected; }
>!</code>
>!
>! So upon running the code, you'd see
>! 
>!     `Troll detected`