Absolutely, but I think if you asked him he'd say he puts a hard line on it but with a heavy conscience admits he's had to cross it. By and large, I think killing is so much of a last resort Clark never actually considers it until he literally has no other choice. He's more or less evolved into the patron saint of second chances, so it really does have to be extinction event level for him to even get the notion else it kind of flies in the face of what he's about currently.
Hell, deep down he probably still wishes Lex would get it together and save the world. I don't think he believes it will happen, but it's probably held him back from killing Lex more times than not.
When did I say it was Waid who did it? I said that with the creation of the Speed Force, the engine for Flash becoming infinitely faster than Clark was created. It gave him an infinite reservoir to draw on. Not too long after you had Millar writing Wally outrunning the metaphysical construct of death to the end of time to prove it will never catch him (or Linda) until he allows it, thus getting it to spare his wife. It was only a few years later. Much like how Tower of Babel was where everyone extrapolated Batgod and Watchmen/DKR where everyone learned to masturbate over dark storytelling.