how could captain America save the earth from the hulk, aliens, travel into space, but die from a bullet?

His writer got tired.

by piercing his skin and losing blood, something that is vital to all living creatures if they want to live

It was a really great bullet

It was actually multiple bullets.

He was still human (an enhanced human, but still human), which means anything that could kill a human could kill him (it might take more time or might take multiple times to do it, but that's besides the point).

because it's dramatic and not expected

the readers of Marvel comics all collectively gasped in shock and some were even moved emotionally by the death of the man who stood for everything good about America

what better way to kill a superhero than have him die like any ordinary human being

the shooting death of Captain America showed the bleak truth to most superheroes like him, they're still human, and they're very much mortal

remember, Marvel Comics had to compete with previous Superhero deaths, something DC dominated with the Death of Superman 15 years ago, killing the man of steel himself is no easy feat, so Marvel opted for the simple yet dramatic killing, and it worked wonders

but don't worry, Captain America will be back before you know it, and there will probably be a complex story regarding his survival

Because he's not dead. Let's face it, they brought Bucky back from the dead, what are the odds of Cap staying dead?

Nope. It was a hoax. Or a clone. Or a skrull. Or something. We'll find out in a year or so.

because marvel comics needed a sales boosting publicity stunt.

i assure you he will be back among the living in no time-nobody in marvel comics stays dead for very long. (just ask jean grey, collosus, the punisher, aunt may or any other character thats ever been killed off!)

The greed of Marvel Comics for a big event book, that ultimately in a few years will mean even less than:

1) The Death of Superman
2) The Death of Captain Marvel
3) Batman getting his back broken by Bane.
4) Somebody else being cap (during the 1960's/70's/90's and thanks to a recon the 1950's.
5) Hal Jordan dead.
6) Bucky Dead.
7) Aunt May Dead.

All sound and sight signifying nothing.