As for Superman killing: Back in the Golden age (1930s-40s) Superman had no qualms about killing bad guys, though he was still a beacon of moral virtue and so forth. Is that what you mean, or were you more into a 'Clark as a villain' concept?
Neither really. Clark´s motives are up to the writer.Point here is that in Elseworlds setting Clark just might choose another values.
1. He could be outright "villain Clark"
2. He could be 1938-style Super-anarchist
3. He might condemn the System as corrupt and Kents as naive.
4. He might consider it impossible to be Superman and respect the law at he same time. "Even if i just go running at superspeed, they might prosecute me for reckless endangerment".In comics this is always ignored.How safe should Joe Average feel if there is a man running at hypersonic speeds. WE know there will not be unfortunate collisions.What would Joe Average believe ?
5. Then there is whatever motives writer can invent.It is up to you.