Nice people tend to gain lots of respect from others, so when they get mad, people tend to fear them a bit because they have so much respect for them.

Another reason why nice people seem "scarier" when they go nuts is you're so used to that nice version of them that you're unprepared to react to the angry version. Since you hardly expect them to go mad, you don't know how to deal with when they do. It's out of your power, so it makes you frantic. However, this only applies if you're a jerk who likes to pick on nice people because jerks usually have this wrong perception that nice people are easy to walk over.

I think that’s anyone who hold things in and are not very confrontational when it comes to things that upsets or bothers them. 🤷🏽‍♀️ Nice or not, you’ll eventually blow/explode and it won’t be pretty.

Oh yes because they let things build up and then when they go off it's like a nuclear explosion. Also it's usually so unexpected.

Ya, when a patient person finally loses their patience...thats it

Not necessarily.
Most nice people just politely distance themselves,
or ease away so slowly that it's not noticed,
or practice skillful avoidance.

However, people who suppress anger for a long time may explode in a way that seems out of proportion to the apparent trigger.
And for the person on the receiving end, the sudden explosion may seem out of character.
This usually means they were insensitive to the more subtle signs that were always there.

Yes, because

1. It takes people by surprise when a nice person shows their anger, and

2. They have so much anger bottled up, from holding everything inside and being nice all the time, that when they can no longer hold it in anymore, it's huge, like a volcanic eruption.

And yeah, that sounds like me. I did that with someone last February. But it didn't do any good. All it did was get me unfriended and pretty much ignored since then. :(

Nice people think their doing a service by always trying to appease others and (going with the flow), that they don't know their actually doing a deservice to themself and to others by bottling up how they truly feel inside until they explode and now everyone sees them "for who they truly are" but really they're only seeing the you that accumulated all this stress unknown to others upon urself

Expectations

Its because your behaviour is out of character as far as they know.. Suddenly they are wary of you. I know people who curse like its punctuation and no one notices. I let one word out and I get attention.

Yup. I remember the look of their faces when I finally snapped. Shouldn't have apologized for it though.

Probably because they hold themselves in check, the raw emotions are beyond their control

I think folks are just not expecting it, so, it looks "scary" to them

Don’t know but I think they should be..

No, because you know where you stand with them. They tend to be grounded with morals and are consistent because of that. And, that's something you can count on. That's not scary.

I dunno, I think everyone is pretty scary when they've truly had enough.

Loud and obnoxious people are just show offs it's the quiet ones you have to worry about

Theyre smarter and more capable than naturally mean people which can be intimidating... but scarier? No. They're like that because they have morals and rules to hold themselves back.