Since you asked for our opinions on the poem, I'll offer mine. I'll look at it as how McVeigh may have possibly saw it from his viewpoint...
INVICTUS
Out of the night that covers me,
Black as the Pit from pole to pole,
In poetry, "black" and "night" are often used symbolically to represent death.
I thank whatever gods may be
For my unconquerable soul.
Whether McVeigh believed in God or not, he acknowledges that his soul is "unconquerable". The guy is now part of history, and he will live on for as long as the memory of the Oklahoma bombing remains.
In the fell clutch of circumstance
I have not winced nor cried aloud.
Under the bludgeonings of chance
My head is bloody, but unbowed.
McVeigh knew what he was doing. He wanted to die. He accepted whatever consequences befell him. Although we may have 'bowed' our heads under the pressure, McVeigh assures us that he has not.
Beyond this place of wrath and tears
Looms but the Horror of the shade,
And yet the menace of the years
Finds, and shall find, me unafraid.
In other words, he's not scared (or so he wants us to believe) of what is about to happen.
It matters not how strait the gate,
How charged with punishments the scroll,
I am the master of my fate:
I am the captain of my soul.
And so he accepts that what is about to occur is the responsibility of him alone. He was "charged with punishments" from "the scroll" - in other words, he was charged with willful murder, convicted, and punished with the death sentence.
McVeigh is stating that he accepts the verdict (it was what he wanted all along) and so beyond the walls of death, he is still the victor, having accomplished what he wanted to do.