To be fair, Porky has a COUPLE redeeming factors in 3.
All right. I'll entertain you.
In some twisted way, he did consider Ness as a friend in the end. He does have multiple mementos to the boy he constantly antagonized. However, the Minches upbringing made him believe that the only way to gain friends was to belittle, condescend, and in general prove that he was better than the ones he wanted to hang out with.
I can't argue that the Minch's upbringing was poor, considering it's heavily insinuated they're beaten when they do something wrong, but a lot of Porky's character traits came from his father: he's greedy, he's condescending, he feels that he should be considered better than everyone else. All of these character traits are obvious in Mother 2/Earthbound, and also still present in Mother 3, so that makes you wonder how much genuine influence Giygas had over the kid from the beginning. The downward spiral was probably sometime after he crashes the Monotoli copter.
Also, yeah, Porky does consider Ness a friend, but the kind of things that he has of Ness's and the 'memories' of a trip he was never actually a part of means he was pretty much obsessed, to stalker levels. A lot of the things you find in New Pork City that refer back to Mother 2/Earthbound are, for the most part, all things Ness touched or was a part of. Porky had nothing to do with it. That's stalker behavior.
It can be argued that all Porky wanted was to be loved, which would explain the robotic duplicates of his own mother and the brainwashings to make people love him.
The brainwashing that was going on in Tazmily Village was more to placate the populace. The robotic duplicates of his own mother would definitely be more indicative of his need to be loved, so I can't argue there.
Finally, asides from the chimeras, A ton of Porky's changes to the islands seem to be made with the purpose of recreating his home, owing to how he is trapped in that era. It's only when he realizes that he can never go home again and that nobody would ever willingly love him that he goes the route of destroying the world.
Not saying that excuses his atrocities. But hey, at least it explains them.
These are all still conscious decisions he makes, though. He consciously decides to invade an entire island, consciously decides to screw up nature, consciously decides to experiment on helpless animals, consciously decides to ruin the lives of other people, consciously decides to destroy the homes and people that won't willingly be placated, consciously decides to casually end the world when he discovers he can. They're all conscious decisions made by someone who's clearly of the same right mind and mannerisms as when he left his own time, and all the things he undoubtedly did across time were all conscious decisions, as well.
We can't chalk up or excuse any decisions he makes in Mother 3 because, unlike Mother 2/Earthbound, there's no evil influence.
Porky acts the same in Mother 3 as he does throughout the entirety of Mother 2/Earthbound. He's a genuinely villainous person, and if Giygas had any real influence in Porky's transformation from unassuming brat to genuine villain, it was simply to push him in the right direction for it.
And hey, I'm probably totally wrong and Porky was just bad, plain and simple. But it lends credence to the idea that Porky Minch is one of the greatest, well-developed Nintendo villains of all time.
At the end of the day, a lot of it's theory crafting, but all the evidence points to Porky doing everything he does in Mother 3 simply because he can. Heck, based on what we see in Mother 3, it insinuates that a lot of what Porky does in Mother 2/Earthbound
is his own conscious decision to do. He even admits later on in the game that Giygas is clearly not in his right mind, so it actually raises the inevitable burning question: was it Giygas's influence that took advantage of Porky and pushed him down the path of villainy, or was Porky simply taking advantage of a psychologically crumbling Giygas to put himself into a position of power? The latter certainly seems possible.
tl;dr: Porky is a villain 'cause all signs point to him doing the things he does simply because he can.