9. A Kryptonian Dragon
Most people needn't worry (though tell that to the populations of Metropolis and Smallville), since it appears Superman keeps most of his killings solely to creatures from his home planet. He's probably jealous of people muscling in on his "Last Son Of Krypton" shtick. How can he cry about how alone he is in the universe if he's actually not alone in the universe? Jeez.
Keeping with the grittier style introduced in Man Of Steel, DC Comics recently relaunched their whole shared universe under an initiative called the New 52, which has mainly been about eliminating interesting character development and making everyone angsty murderers.
Superman is no exception. A recent storyline in the Man Of Steel's main title saw a mysterious Kryptonian villain - but not Zod, mind you - trying to resurrect his homeworld, but on Earth. And apparently Krypton had huge dragons, because if it can have Russell Crowe, why not huge dragons? Naturally, it's Superman's mess to clean up, so he hot tails it over there and starts putting the smackdown on this mythical fire breathing monster. He comes to the conclusion that the creature's DNA is decaying, so it's not technically alive, but it is hella destructive.
So he roasts it with his heat vision and it explodes. Sure, there's the "not technically alive" proviso and the dragon did have the potential to harm civilians, but also Superman just killed a sentient being without really thinking too hard about it. And an endangered species, at that! Kal-El, Bill Oddie would have words with thee.