I think the writer must be very bravo and ingenious for write a story where Superman is a killer!
Clark is a complex person with so many values! His moral integrity is so strong and I think it's difficult for any reason to change this fact! Maybe he could be more "angry" and more "bad" with the right situation but a killer? I don't think so!
In the HoL he tried to safe Luthor at the end but he didn't have superpowers for the kryptonite! He hate Luthor, he knew who was Luthor but he was the same sad!
Maybe Clark could kill but for error! It's a good explanation! The only one for me!

CLARK: I hated not being able to tell you. I mean, you think it was easy watching you swoon over Superman and ignore me?
LOIS: That doesn't make any sense! You are Superman!
CLARK: No, Superman is what I can do, but Clark is who I am...