As for that cobra - I always thought the idea behind that was to prove how evil Lex was.
If you read the script online
here (page 178), you'll find the following exchange.
Superman walks back onto the terrace, Luthor following. As
Luthor watches, Superman begins a slow vertical ascent.
SUPERMAN
One other thing. If you ever need
to find me, all you have to do is
look _up_
Luthor does, indeed, have to look up as he watches the Man
of Steel go ever higher, then disappear into the clouds.
Luthor turns, walks back into his study, his anger
building. At that moment, Asabi enters carrying a wet
suit, oxygen tank, and harpoon.
ASABI
The tank is ready sir. The Mako is
a particularly fine specimen.
LUTHOR
Send it back to the aquarium.
ASABI
Sir?
Luthor slowly walks to his desk, sits down in his chair,
swivels it toward the lights of the city.
LUTHOR
I have bigger fish to fry.
The part about Asabi, the scuba gear, and the shark didn't make it to the screen. But this shows that Luthor's intent was to test himself against every conceivable opponent - until Superman showed up and became his number one obsession. (Lois, of course, supplanted Superman in Lex' personal obsession list in "Pheromone, My Lovely.")
I don't know why. Either Lex was a compulsive sociopath or he just felt lonely and needed a really good hug.
