What's a red flag in someone?

They pour the milk before the cereal.

I said red flag, not sign of a psychopath
Damn. Am I excused if i have only so much milk left and do it to not end up with dry ass cornflakes?
- Then and only then. But it should pain you to do it.
- No, add small amounts of water to stretch it like a normal person.

Red Flags (if I don't blatantly show disgust, I'll probably ghost you after observing this stuff):

Littering
Treating animals like objects
Treating staff in restaurants and shops etc. like trash
Being into manosphere shit or treating any podcaster / influencer / asshat with a microphone like they're a guru
Interest in conspiracy theories and/or 'free speech' aka actions without consequences
Crying about wokeness or women or queer people

Amber Flags (I'm a bit put off but I don't want to judge a book by its cover so I'll get to know you first):

Subscribing to a religion
Having no neck, a red face, short / no hair, tribal tattoos and walking like you've got an invisible water bottle under each arm
Being a constant 'positivity' person

Edit: I feel like that's a lot. Maybe I just hate everyone lol. Maybe I'm the red flag.

Conspiracy theories is a good one, it can be a sign of serious MH conditions. Some theories border on clinical paranoia
- Yeah and sadly the conspiracy theory scene seems to be completely taken over by bad actors with right wing agendas. That prey on these people who actually need help, rather than to be fed stories about George Soros and the deep state and microchips in vaccines etc.

An adult that often doesn't accept when they're wrong, glosses over it or shifts the blame. Yes, it's part of growing up but past a certain age it's a terrible sign in someone.

Such person is unfit for marriage or friendship
That's not a red flag that's a deal breaker.

You mean the first things to ask a new acquaintance aren't the names of their childhood best friend, first pet, and maternal grandfather?
- If you send me money I will explain why

Unnecessary aggression or assholery.

Mocks others, gets angry at jokes about them.

Being religious.

It’s kinda sad to put it that way, but I’ve found almost universally that devoutly religious people are borderline incapable of actual critical thought

Fatal, depending on point and depth of penetration.

Good one dad@

Kisses their dog on the lips, but does not french the cat.

They don't listen to music at home.

I don't specifically have any. I just trust my feelings and am open and honest as possible.

Their bathroom is destroyed. Usually a bad sign.

A green flag is seeing a stuffed animal or two in their bed.

Hard disagree on that green flag. In my experience is not an indicator. Both the worst people and the love of my life have had stuffed animals on the bed.
Does it count if my stuffed animals are in the living room?

They accept a lack of evidence as natural in vetting someone for their supposed wrongfulness.