If he never attacked Helgen, the Dragonborn would've died right there and he would've been able to take over Tamriel. So...was he [censored] or did he not know that Dragonborn was dragonborn? Heck, why did he attack Helgen of all places anyways?

This is disturbing yet highly intriguing...

The reason he attacked Helen is because as alduin left the time wound, Helgen was right in front of him and he probably senced the dovahkiin there.

He probably wanted to kill the dragonborn but accidently saved his/her life instead.

Alduin is a selfish dragon that gets possessive over his prey. Alduin isn't going to let some weakling imperials touch his prey. It would be like an insult to him.

I think he was trying to save Ulfric Stormcloak. Keep his enemies fighting each other. Makes his job so much easier.

I sort of agree with this claim. I just believe that he was prolonging the suffering that Skyrim has already endured by saving Ulfric. Remember, evil beings are EXTREMELY arrogant. They usually like to toy with their "pray" before the end. Maybe Alduin wanted to see the war more extended and bloody than it would have been. I believe (having many evil characters) that he simply wanted to see the world suffer before it died, and weather Ulfric knew it or not, he was hurting, not helping Skyrim.

And then Alduin is defeated and Skyrim wins its independence.

That meat puppet....IS MY MEAT PUPPET. HANDS OFF EXECUTIONER MEAT PUPPET!

I think so, too. He had just been in a fight with some dragonslayers, sent through time, enraged and confused (and probably hungry)... He saw a town before him, some petty humans, sensed the presence of a dragonborn and attacked.

He couldn't even have known there was a civil war going on.

Think about it this way. Alduin's just been sent forward in time after duelling with some Nord heroes. His prophecized return is finally complete.

Of course he's going to attack the nearest village.

I've thought of this myself but never came to a good conclusion.

My guess is that since he was trapped outside of time,he saw that man and mer were stronger and his kiin were dead. And then he had a thought; "I can't conquer the world fast enough all by myself.I need to resurrect my homies."
Plus, the Dovahkiin is like on par with strength or stronger than Alduin.An equal adversary.So Alduin needed an army.
He may also have had visions of the future and saw his own demise,but he wasn't going out without a fight.

He was just saying hello. Bringing down meteors is "hello" in draconic script.

Nonono, Paarthurnax says a Fire Breath to the face is good enough. Meteors are for pompous creatures who want to show off. ^^

In fact, it must be quite hard to be a dragon and strike a conversation with an ally. Every word you say causes quite some chaos. One moment you're igniting things, the next you blow everything away, and the other you're throwing lightning strikes and meteorites at everyone. Poor dragons. They're actually only severely misunderstood, they just want to say hello and everyone goes hostile on them.