To satisfy all the conditions: I came with this:
I think this is
I pinch and bite
Sword pinches on touching the sharp edge and bites when it cuts the flesh(sorry for being so much descriptive).
For dark and light,
It helps in the dark or light to battle or to fight for the safety of ones kingdom.
I rust and gleam,
Rusting occurs to it in the course of time. It gleams once it is clean before the battle.
Divide and team,
It divides the people, empire by being in the hand of wrong people, but some time unites in to a single kingdom.
I'm your enemy, and your friend,
It can be enemy in the sense, if it is in the hand of our enemy or being misused in killing people. Even it can kill us if misused. However, it can be a friend if we know its real purpose, i.e to eliminate the abominable.
I'm your beginning, and your end.
An empire starts on winning a battle with the sword-> beginning. It ends the other empire-> the END
On the hints.
Made my nature abused by men
Whatever is required to make it is present in the nature. Abused in the sense of lust for power and extending there empire by humans.
Made in peace but used in war
It is made in peace for the safety of kingdom, but used in war for protection of the kingdom from the enemies.
Now used as a showcase
Not very difficult, we have them in the museums. Famous swords of the histories.