At the start of the fic he wouldn't hurt a fly and is weak, but as things progress he gets his hands dirty and ends up becoming more and more ruthless and becomes stronger.

I enjoy the gradual progress of watching a main character become his worst nightmare.

Has to be male and it doesn't matter what fandom. He could be a SI or a OC. ALSO NO ANGST. Just don't recommend me Worm, if I have to read any more Taylor being angsty and shit I will seriously put a bullet in my brain.

For example: https://www.fanfiction.net/s/6942921/1/He-Who-Fights-Monsters

 

If you don’t mind some classic Chinese Xianxia, I Shall Seal the Heavens has the kind of protagonist you’re looking for. He starts off as a student of the Confucian arts looking to gain a government position through the exams, but becomes something more and eventually has to set aside the Confucian values he held so dear. Slight bit of angst but nothing overwhelming.

 

There is Going Native by Ideas guy, its a OC insert into a crossover of DC and Dragonball, where a human OC is inserted into Vegeta's brother Tarble and like the title suggests hes going native over time and becomes more ruthles and saiyanlike as the story continues.
Its in the Library, https://forums.darklordpotter.net/threads/going-native-dbz-dc-comics-by-ideasguy-m.39204/

 

Well, I don't know why I just got called out, but seeing as I'm here I might as well offer some suggestions:

1. On the Way to Greatness - Harry Potter gets sorted into Slytherin. An edgy bildungsroman, but quite good.

2. Son of Suns Trilogy - Luke Skywalker is captured and brought to Coruscant and gets turnt into an sith boi. Lotta angst tho.

3. ??????

@Majube you wanna jump on this? (Yes I'm continuing the call-out train, what of it).

 

I'm not sure I'd call this fic good, and it's quite preachy at points, but Brutal Harry fits your requirements. It's a weird fic...

 

Arguably The Merging by Shaydrall.

Don't think Harry starts out being unable to hurt a fly though.

 

https://www.fanfiction.net/s/9057950/1/

-good and fits what you're asking for but the writing is a bit sloppy

this one maybe -it starts off with mc snapping, https://www.fanfiction.net/s/5866937/1/Control

https://www.fanfiction.net/s/13022632/1/A-daring-synthesis

https://www.fanfiction.net/s/7996957/1/The-Asking-Price -short

Some of these are pretty angsty but ¯\_(ツ)_/¯ was pinged on short notice @Paradise prolly knows better ones

 

So this is exactly what you are looking for:
https://www.fanfiction.net/s/6942921/1/He-Who-Fights-Monsters

Rosario + Vampire (you don't need to know it at all, summary: secret monster high school where monsters learn how to be human, and a normal human accidentally gets sent there). In the original, he befriends one of the most powerful monsters and builds a harem. Things go decidedly worse for him in this fic.

 

Ahem.

(whole of escalation in this one for a number of folks, and the author tries not to hold back on the ugly side-effects of all of it)

 

Mother of Learning somewhat fits that definition from what I remember of it, although I don't think Zorian (the MC) was ever a "wouldn't hurt a fly" kind of person.

It's a groundhog-month fic, with fairly steady power increases. Zorian definitely loses a fair amount of his morals along the way.

 

Lol, they put that in their post.

Anyway, if we're including web serials I'd say Defiance of the Fall

 

There's the excellent A Practical Guide To Evil. It's not a fanfic, instead it's an original web serial. The main character is female, but it really doesn't have any real impact on the story. If it's the 'making goo-goo eyes at hunky males' aspect that you don't like, the main character is bisexual and when romance does enter the picture it has been with other female characters.

(I'll gladly take any chance to plug this story.)