Mario is Supporting an Evil Monarchy
Let's make this clear: there is really no good option for the Mushroom Kingdom. They're either being ruled by the Toadstool family or by self-proclaimed "King" Koopa. Both of their options are an oppressive monarchy. Really no one's up for democracy at all. So it shouldn't really matter that Mario's trying to re-instate one ruling monarchic family over another, right?
Wrong. The Toadstools are the evil ones. Hear me out: all we know about Koopa is that he very recently took over the kingdom so recently that it's very unlikely he was able to build any castles himself. So what we're left with is the reality that in all likelihood it was the Princess' family who built those lava-filled, nightmare castles. It would also explain how secret pipes were installed in all of them, seemingly without Koopa's knowledge, and why Koopa stands atop a bridge that can be instantly destroyed simply by touching an axe. If Koopa had built these castles, odds are he wouldn't have made his favorite bridges instant deathtraps.
Beyond that Koopa has the will of the people behind him. His supporters number in the thousands! The people supporting the Princess includes two destructive, piss-poor plumbers and some mushroom-headed weirdos who hang out in horror-castles.
Mario is an Insane Murderer and Princess Toadstool Couldn't Care Less About Her Subjects
"But wait!" you argue. "There are people who support the Princess! But they were turned into brick blocks by the evil Koopa!"
First off, don't argue at your computer screen. It will not respond to you. These are pre-written words, dude. You're weirding out everyone in the computer lab.
Second, good point, hypothetical reader! There were untold numbers of denizens turned to brick, at least according to the game's manual, which is a fun weird horrible fact that has become more popular and well-known in these modern internet times of ours. So maybe the noble Toadstool family was looking out for the best interests of the people, but that crooked Koopa got in the way of their benevolent love for the commoners.
Except not at all. For one, Mario destroys everything he possibly can, mostly those bricks. He's rewarded for it. He does it for points, for coins, and sometimes to kill Koopas and Goombas unlucky enough to find themselves beneath the bricks Mario has targeted. So Mario pretty much murders the entire population of the kingdom, while at the same time tries to save the oppressive monarchy by crushing as many animals as he can find. Great.
But the best thing of all: reinstating the Princess does nothing, since the bricks continued appearing in Mario games to this day. By now, Mario would have to be aware of the bricks' true nature, but he doesn't care. And the Princess certainly doesn't care enough to bring them back to life either according to the game's manual, only she has the power to bring the people back, but she never actually does. In fact, the only transformed people anyone ever really changes back to normal form are the kings of each world in Super Mario Bros. 3, while the regular people are left to be destroyed by a chunky plumber in a frog suit.
What a surprise.
*And beyond this, the entire theory must come to question if Koopa really had the ability to turn people into bricks, why wouldn't he have done the same to Mario, instead of jump around and shoot ineffective fireballs at him? Why wouldn't he have done it to the Princess, instead of letting her sit around behind him? This seems like an incredible power to just waste on everyone except the people trying to murder you and your entire army.
It's All a Game To the "Heroes"
So Princess Toadstool is a probably oppressive monarch who doesn't care enough to bring any of her subjects back to life even though it's well within her power to do so and has had some part in constructing the most horrible castles known to man and gave a sweet plumbing gig to her idiot boyfriend and ruined the sewage system for the now-bankrupt kingdom forever. That's the starting point of why the Princess is equally messed up and evil, but here's the rest.
Princess Toadstool is barely captured. She's not tied down, trapped, or otherwise being put in any position to stay where King Koopa tells her to. She's standing behind Koopa, waiting for someone to come by, leaving her with three options:
- Sneak up behind Koopa, touch the axe, and murder her supposed kidnapper, freeing herself and the kingdom.
- If she can't climb or jump the wall to get to that position, she can still walk out the back door. Every castle has a back door, as Mario demonstrates at the end of every previous level, after being told by Toad that the Princess is in another castle (and speaking of Toad(s), all of this applies to him(them) too).
- Stand there like a doofus for hours until your dumb boyfriend curbstomps his way back to you. (WINNER!)
So there's no reason for Mario to have to go through all this crap to get to the Princess. She just doesn't feel like moving at all, leading to a kingdom-wide genocide. Then to add insult to injury, she gives you a half-hearted "thanks" and orders you to start a new quest. In fact, this all seems like it was an elaborate game for her and Mario she hung around and let everything turn to shit just to see if Mario could actually get to her. Everyone else were just pawns, there for her amusement.
So in the end Mario, the shitty greedy serial killer plumber, starts over, doing the same thing he just did. Princess Toadstool will be reinstated to continue ruling over a bunch of bricks she couldn't give two shits about from her terrible lava castles. Untold numbers of innocents are dead. The economy has collapsed. The kingdom is in ruins
And, maybe worst of all, flushing a toilet still won't do anything.