system
Given the following post, respond as if you're commenting on Reddit:
human
[BvS] Why would Luthor try so hard to kill Superman if he believed more aliens are coming?

For most of the movie I thought Lex simply didn't see Superman as necessary (anymore) but a potential threat, but after he went to prison he started rambling about Superman's death being a dinner bell for other aliens, so why the hell did he do it at all?
gpt
Because he's batshit insane.  

Or, to be more precise, he's been *driven* batshit insane by Superman's very existence.  Superman's presence and deeds fly against almost everything Lex believes in.  Most importantly, Lex believes that Superman cannot possibly be a benevolent entity, and so must be eliminated, whatever the cost.  

This is why Lex manipulates events so that Batman would fight Superman.  He's got files on a bunch of metahumans on Earth, but he picks the Gotham vigilante because he thinks the Bat's got the best chance of victory, and because Batman's gradual descent into darkness had made him the easiest to manipulate.  But when Batman failed to kill Superman, what was Lex's backup plan?  To unleash a super-destructive unkillable Kryptonian monster.  

Let's just think about Lex's Doomsday plan here for a second.  What was Lex expecting to do after Doomsday killed Superman?  Just let Doomsday run free?  He's got no way to put the cat back in the bag.  Doomsday would quite possibly lead to the end of the world, and someone of Lex's intelligence had to recognize that. But he didn't care.  As long as Superman was slain, Doomsday could bring about Armageddon for all Lex cared.  All that mattered was that there would be no more Superman.  

So yes, Superman's death will leave Earth far more vulnerable against future threats from the cosmos.  But Lex doesn't care.  Superman is dead, and so now Lex Luthor is content.