TV Tropes Will Ruin Your Life.

And so will Pokémon.

... But that's okay. :)

Found this on the Pokémon WMG (Wild Mass Guessing) page:

The main protagonist of Pokemon Blue and Red is a twisted, sadistic freak.

He captures Pokemon and, after capturing them, records what he learns about them onto his Pokedex. Since he can only do this after capturing them, he must learn what he writes in the Pokedex entry from the Pokemon he owns. However, take a look at some of the entries this 'hero' records:

Raticate - "It uses its whiskers to maintain balance. It apparently slows down if they are cut off."

To find this out, Red (or Ash, or whatever you want to call him) must have got a knife and hacked off the poor thing's whiskers just to see what would happen.

Golem - "Its boulder-like body is extremely hard. It can easily withstand dynamite blasts without damage."

So Red has tried to use dynamite to blow up his own Pokemon. Oh, and there's more. Red is either extremely dedicated to scientific enquiry or just plain likes to hurt things. Therefore, Red's rival is the good guy trying to stop Red's mad experiments.

• Considering that Professor Oak gives him the Pokedex and sends him on the journey in the first place, Red is working as his lackey, probably to keep Oak ahead of his peers in "the field of Pokemon research." Whether Oak condones his more... violent experiments is debatable, but it wouldn't be surprising if he does.
• So, in effect, Red is Oak's Psycho For Hire?
• And expanding on that, the data Red collected made Oak so successful that other researchers across the world are adopting similar tactics, recruting children both for their ease of manipulation and less restraint when it comes to harming Pokemon For Science.
• This might also explain why Red becomes a hermit in G/S/C; his experiments were discovered, and he was forced to go into hiding to avoid prosecution.
• Oh god! this explains everything. And Red won't speak because he felt extreme guilt for the sins he commited and now can only scream in fear.
• AAAH! My childhood!

This evidence is from Yellow...

Staryu - As long as the center section is unharmed, it can grow back fully even if it is chopped to bits.

...The man must be stopped.

Slowbro's Yellow entry: "Lives lazily by the sea. If the Shellder on its tail comes off, it will become a Slowpoke again." Since Slowbro do not regularly devolve back into Slowpoke, we can only assume Red actively ripped this Pokémon into two pieces.

Shellder's Fire Red entry: "It is encased in a shell that is harder than diamond. Inside, however, it is surprisingly tender." Red, you sick, sick bastard...

Ooh, and we see a bit of daring do in the RS entry for Kangaskhan: "If you come across a young Kangaskhan playing by itself, you must never disturb it or attempt to catch it. The baby Pokémon's parent is sure to be in the area, and it will become violently enraged at you." What was he trying to do, steal the baby? Or maybe the smaller ones are easier to euthanize and dissect?

Alternatively...

The protagonist of Red/Blue/Yellow was a lazy kid that made everything up.

More pleasant & frankly more plausible explanation. The dex entries are written immediately after a mon is caught, no way he actually took time to study them. The entries are lies he made up on the spot so he could go back to his fun adventure because he's you know, ten! Did the grown-ups really expect him to waste a perfectly good extended vacation on studying? Also this explains some of the more curious heights and weights (Snorlax sure doesn't look 6'11").

Don't get me wrong, I adore Red to bits, but this was just too hilarious to pass up. 8D