When the male protagonist meets Zao, after serving in the military and having their entire world destroyed by the chinese, Kill Zao or forgive him?

Well Nate would do whatever the player chooses. But realistically, it depends. Bethesda kinda painted the protagonists as hearts of gold, so if he gave Zao a chance to talk, he could probably sense the regret in Zao. So yeah, he'd probably forgive him, but then again, how the protagonist acts isn't set in stone.

I also find the dialouge from the main protagenist a bit unlikely.
The main character usually is seen as a good guy and like @"Tactical308" said the protagenist as how bethesda wanted him to see would never say "die you commie bastard"

Thats what i really dislike about fallout 4.
I absolutely loved how i could just straight up attack people in 3 and NV by choosing a dialogue option. And in NV i could even scare the shit out of them and make them run away.

I think he wouldn't... The main character's catch phrase is supposed to be " War Never Changes "... He hates war as much as a pacifist... So I think he wouldn't hold any grudges against a Chinese officer who never launched his Submarine's nukes to America

Just because he was a soldier doesnt mean he hates the Chinese. He could have a hatred for them as a result of the war, but I sincerely doubt he would hate a kindred spirit like Zao. A soldier from his time who remembers the war and can look back and be like. Damn was it worth it.
I dont think Nate would kill him.

WWLPD?

Kill the communist scum

200 years later, you emerge from the vault by yourself and you see what the War has wrought on the land. After seeing the devastation, it's hard not to see how pointless the War was. Everybody lost.

However, Bethesda did include an option to call him a commie and get him to turn hostile. He also says in chinese when he turns Hostile, roughly translated, "You idiot capitalist, now go die"

So it really is up to the player. The benefit though of helping him nets you some tactical nukes to use.