These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Brainiac. He brainjacks Luthor in the most horrifying way possible, murders most of Superman's remaining friends, and is completely devoid of anything resembling a conscience.
Mxyzptlk is far beyond Luthor, Braniac or even General Zod in that he's an Eldritch Abomination, practically an Evil God who was going to wreak havoc onto the whole universe if Superman had not stopped him, and the only way left was to put him down permanently.