Why did that General guy kill himself in the beginning part?

Firstly, a minor point; Fambrough was a major, not a general.

Fambrough seemed to have mental heath issues. We hear the clanking of bottles when he opens his desk drawer, suggesting alcohol issues. His staff at the office seem used to the major's behavior and mostly ignore it.

HE was a Major

HE was a Major

I'd say he was already dying an alcoholic and severely depressed with little hope left. I don't think speaking to Dunbar had a huge effect on his actions, he had probably thought about it many times before and decided that now would be as good a time as any.

It's in the novel. Essentially, he had slowly gone mad. He didn't want to be there, he was alone, was out of his element, and had no friends.

It's primarily a plot device. No one knows that Dunbar is at the fort because the one officer who knew killed himself. The soldiers that had been at the fort were starving and left on foot to try to get back to town, but were all killed (to my memory) so no one even reported that the fort was unmanned. Thus the long time without resupply or contact.

In the novelization, it makes way more sense but putting it in the movie would slow it way down and it's already a slow movie.

Maybe they'll make a shitty prequel (on streaming) that explains it for people who can't come up with their own explanations.

According to Wikipedia https://en.wikipedia.org/wiki/Dances_with_Wolves and the Internet Movie Database (IMDB) https://www.imdb.com/title/tt0099348/?ref_=nv_sr_srsg_0_tt_7_nm_1_in_0_q_Dnces%2520withwolves the character was Major Farnboough.

In movies it is fairly easy to tell the rank of US army offices, especially if they are not wearing modern combat camouflage uniforms that make it hard for enemy snipers to recognize the officers.

Here is a link to an illustration of US armed forces rank insignia.

https://www.defense.gov/Resources/Insignia/

In the 19th century the insignia for officers was the same except that 2nd lieutenants had bare shoulder straps without ran insignia.

In modern times the the rank insignia is metal and placed directly on the shoulders except that on the full dress Uniform the insignia is on shoulder straps. In the 19th century the full dress officer's uniforms had the insignia on epaulettes and the duty uniform had the insignia on shoulder straps.

https://en.wikipedia.org/wiki/Epaulette#:~:text=Epaulette%20(%2F%CB%88%C9%9Bp%C9%99,referred%20to%20as%20shoulder%20scales.

https://www.iragreen.com/army/shoulder-straps.html

According to regulations at the time company grade officers (2nd & 1st lieutenants, & captains) coats had a single row of buttons, while field officers (majors, lt. colonels, and colonels) had double breasted coats with two rows of buttons. Logically general officers (brigadier generals and major generals) should have had three rows of buttons, but had two rows like field officers. But each row of buttons on a brigadier general's coat had 8 buttons in four groups of two buttons, and each row of button's on a major general's coat had 9 buttons in three groups of three buttons.

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