I'm looking for manga where the protagonist looks normal but when he snaps he is real psycho who gives you the chills.

There are quite a few of these. Not sure which genre you want...

Seinen
MPD Psycho Main character has a murderous alternate personality.

Dogs Bullets & Carnage Main character has freaky fits of rage that are set off by the smallest things. He bit a guy's cheek off. That's flipping psycho.

Black Lagoon The main girl looks like a pretty nice (kinda rude) person. When she's fighting she's flipping scary.

Higurashi Naku No Koro Ni All the main characters seem pretty normal in the beginning. They are not normal. They have legitimately scary outbursts.

Shonen

Code Breaker Main guy is nice guy, but turns ruthless when he's faced with a scumbag.

Looking for more

😀

It will help a bit if you can clarify whether you want a psychological thriller type manga, horror type manga, or more of a romance/drama.

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Image from Gangsta -> Full Image Link http://www.k4.dion.ne.jp/~babyrock/page030.html

mmhh.. isnt there posts like this one?
anw..
switch is a must

** "It's so warm that, Before the snow has a chance to stick, He melts it for me." **

Quote from faraway

Shoujo

Code Breaker Main guy is nice guy, but turns ruthless when he's faced with a scumbag.

This is a shounen, not a shoujo.

This signature was recovered from Hades to serve in my rotting armies.

LOL i was reading Switch when i wrote this, so something in that nature, but if the outburst could be longer i would be happy.

And i will read the mentioned mangas they look interesting.

Mirai Nikki The female lead seems normal but is a psycho
all Higurashi arcs (psycho lolis)
Onani Master Kurosawa might also fit
Tsukihime he normally is a nice guy but turns psycho sometimes

Hanako to Guuwa no Tera Highliy recommended!
It's from the mangaka of Mirai Nikki. The main char isn't a psycho but his job is to treat psychos. Gave me quite the creeps.

Quote from x0mbiec0rp

Quote from faraway

Shoujo

Code Breaker Main guy is nice guy, but turns ruthless when he's faced with a scumbag.

This is a shounen, not a shoujo.

pffftt code breaker shojo? 🤣
but its a really good manga .. maybe bcz is not shojo..

LOL i was reading Switch when i wrote this, so something in that nature, but if the outburst could be longer i would be happy.

i see.. well is not a surprised.. hehe

** "It's so warm that, Before the snow has a chance to stick, He melts it for me." **