Are there any fanfics where a protagonist starts as a weak good guy or normal guy and slowly as the story goes along he keeps comitting more and more acts of violence becoming a anti-hero or villain protagonist.

It has to happen over time. Not be a good samiratan one day and wake up the next day as space hitler.
I want that character development, you know, the juicy stuff.

Any fandom
Has to happen over time - think weeks or months
Has to be male
No gay stuff
No angst/hurt/comfort or pure edge

As examples:

Harry Potter and the Boy Who Lived
On the Way to Greatness
He Who Fights Monsters
Equal and Opposite
Nightfall
Path of Ruin

The reason for their slippery slope should be something selfish, like wanting to survive or learning all kinds of techniques or a obsession, etc.

I want that gradual fall.

 

And my only suggestion was Path of Ruin. *sigh* That is a good "survive into evil otherwise you're dead" story. Too bad he doesn't update as often as he used to.

 

A few recommendations you may enjoy:

Lesser Evils - A fantastic, well written fic. Harry's descent might not be as gradual as you're looking for, but there's definitely a progression and in my opinion, it's one of the better Dark Harry stories out there. The sequel hasn't been updated for a while, but Lesser Evils ends on a satisfying note, so it doesn't really feel incomplete.

The Art of Self-Fashioning - Has a definite progression into darkness, but it might not be as selfishly motivated as you want. Still worth a try, in my opinion.

The Accidental Horcrux - Has a good progression into dark magic, but be aware that the sequel will not be completed and, in my opinion, went a bit off the rails even before that. The first fic is still worth reading though.

What Lies Beneath - Another very well written story. Again, I wouldn't describe Harry's motivation as selfish, necessarily, but there's definitely a descent into darkness.

Hopefully at least some of these are to your tastes.

 

Any Harry/Bellatrix could be considered as Harry slipping into evil.

 

So not sure if this counts but It’s kinda what you’re looking for?

Title: The sum of their parts
Summary:
For Teddy Lupin, Harry Potter would become a Dark Lord. For Teddy Lupin, Harry Potter would take down the Ministry or die trying. He should have known that Hermione and Ron wouldn't let him do it alone.
https://archiveofourown.org/works/6334630/chapters/14514247

 

Traitorous is a good one. Post-OotP DeathEater!Harry, Harry/Daphne.

In short, Dark Magic has an addictive quality to it to a certain percentage of wizards. Harry just so happens to be one, having gotten his first real taste of it at the Ministry at the end of OotP.. He ends up going full blown evil in this one over the course of a school year.