This should be an easy one, as it’s the moral alignment of which I can claim some personal experience. I’ll try and stay unbiased.
Evil is a variable term. There are evil ends, and evil means; those who can recognise that what they are doing is wrong and simply don’t care, and those who simply do not comprehend the harm they are doing in pursuit of some perceived greater good. It can be hard to determine what kind of evil is worst, but both are remorseless in pursuit of their goals, be it because of conviction or lack of empathy.
Neutral Evil is the alignment of the cruel, the driven, those with their own cause to pursue for whom all others are a secondary concern unless particularly useful, or annoyingly obtrusive.
The Ends That Justify The Means
Just to define neutral evil from its close relatives, evil unconfined by law is free to do whatever it pleases, but there’s no denying that it is better organised than those who give in to the recklessness of chaos. Like all evils however, it has a purpose, an ultimate end to achieve, and that end is almost universally centred around power. Money and weapons are means to achieve power, immortality gives you the time to accomplish any plan you put into action, revenge is reclaiming power from those who took it from you. Perhaps you’re seeking a cure to an illness, the power to save another life.
And therein lies the difference. Where Good seeks to empower others, Evil only wants power to itself apathetic to how it effects others, or what damage is caused in the mean time. Take for example the version of Doctor Octopus as portrayed by Alfred Molina, who’s desire to succeed in his efforts to create safe and clean energy led him down a destructive path, stealing money, killing those who stood in his way, rebuilding his miniature sun experiment and nearly destroying the planet in the process. Whether he was driven by an unchecked desire for success, or by a noble cause, his actions were ultimately evil.
This is where the danger of an NE character lies. Conviction to a cause makes them unrelenting and willing to do anything, be it immoral, unpredictable or wholly vindictive. Alliances can only be forged with those whose desires either align themselves or at worst are not likely to interfere. This means that such a character could be brought to a more socially acceptable world-view, so long as as they are not stopped from getting what they want from life… and that thing isn’t the death of everyone.
In the tool-chest of the wicked lie all things unimagined by the wholesome and righteous. Torture, murder, blackmail, exploitation, oppression, theft, manipulation, deceit, and rude words are all at the disposal of those given wholly to the dark side, but that’s not to say that a Neutral Evil character is without principles. To start with, all of the above must have serve the purpose, otherwise it’s just pointless aggression that accomplishes nothing.
Sadism is not beyond an NE character, indeed you may have considered the possible options and considered that viciousness and cruelty are the more desirable ways forward despite their being an equally valid and socially acceptable method, but evil is rarely without cause. Perhaps taking the more terrible path will send a message to others, such as in the pursuit of revenge, not merely having your adversary arrested, shown-up, or their ill deeds made known, utterly destroying them may serve to warn anyone else of what fate might befall them if they cross you. If time is of the essence, cheating your way to the finish-line may be the buy a surplus of time where getting their honestly may not have left enough margin for error.
Sometimes evil is just the natural response, something organic. To take a rather grim example from reality, it has been proven that psychological help, support and therapy rehabilitates most criminals and prevents re-offending. But as a species we cry out for punishment, not sympathy, while logic may follow the proof our emotions demand retribution, and throwing criminals into a hole to suffer for months, years, decades. One of our lesser societal evils.
I find Ultron to be delightfully evil, his programming is complex enough that it can rationalise away any of the laws it may have been originally programmed to obey, driven by the extinction of natural life in favour of a planet more easily governed and ordered, where there shall be no war, only the peace of cooperating machines. While the MCU version may have been a little more egomaniacal (James Spader, you are glorious) than the cold comic-book counterpart, either will go to any lengths to wipe out humanity.
They said no to human trials. The studies had all been going well, tumour shrinkage faster than anything on record, coupled with limited cellular regeneration in affected areas, and then the mice began biting, becoming less social, and finally… Well, a few vicious mice proved nothing, humans had far superior higher brain function, and “mood alterations” is commonplace on side-effect labels. Besides, her mother was suffering, and the vial was just in her bag.
He maintained he’d done nothing wrong. As it turns out people will give you money if you even look like a charity, but nobody took a moment to double check that “Holding Hands” wasn’t just a name he got printed on the hi-vis jacket and stuck on the side of the collection tin, nor had anyone stopped to ask about the so-called charity. A bunch of kids got some over-priced stickers and nobody got hurt in the process, it’s their own fault for being blinded by faux-generosity.
I wanted to cite Scar in this list, but let me give some love to an underappreciated Neutral Evil: Randall Boggs of Monsters Inc. The capricious chameleon (voiced by Steve Buscemi) has no qualms about kidnapping terrified children and harvesting them for energy if it means being the best monster in the company. He’s an ambitious weasel with a vindictive streak a mile wide, and with practical invisibility and Henry Waternoose backing his every play he needn’t fear a thing, except the unpredictability of toddlers.