punctuation - Which is right in this sentence: semicolon or colon?

Daylight began to forsake the red-room; it was past four o'clock, and the beclouded afternoon was tending to drear twilight. I heard the rain still beating continuously on the staircase window, and the wind howling in the grove behind the hall; I grew by degrees cold as a stone, and then my courage sank. My habitual mood of humiliation, self-doubt, forlorn depression, fell damp on the embers of my decaying ire. All said I was wicked, and perhaps I might be so; what thought had I been but just conceiving of starving myself to death? That certainly was a crime: and was I fit to die? Or was the vault under the chancel of Gateshead Church an inviting bourne? In such vault I had been told did Mr. Reed lie buried; and led by this thought to recall his idea, I dwelt on it with gathering dread.
(Jane Eyere)

The highlighted punctuation (after "I might be so") is a semi-colon in Gutenberg eBook, and a colon in Penguin Books. Which is right, or more proper in the context?

Answer

The Purdue OWL says:

Use a semicolon to join 2 independent clauses when the second clause restates the first or when the two clauses are of equal emphasis.

Use a colon to join 2 independent clauses when you wish to emphasize the second clause.

(Those are only two uses for those two punctuation marks; other uses are also discussed and explained.)

So, sometimes, the choice comes down to this: What does the writer want to emphasize?

Here's the original quote:

All said I was wicked, and perhaps I might be so; what thought had I been but just conceiving of starving myself to death?

A simpler paraphrase might be:

I might be a terrible person; after all, hadn't I just been thinking of starving myself?

or,

I might be a terrible person: after all, hadn't I just been thinking of starving myself?

It's a little hard to tell if the second part of this should be "emphasized," or if it should hold "equal" weight. It's a judgment call.

Moreover, there's another possible justification for a colon as well:

Use a colon after an independent clause when it is followed by a list, a quotation, appositive, or other idea directly related to the independent clause.

The "or other idea directly related to the independent clause" part seems pretty open-ended. In this case, the question that follows the first independent clause could fall into that category.

In short, the two punctuation marks have overlapping uses, and the usage guidelines won't always prescribe a clear-cut preference, or indicate that one would be correct and the other would be wrong.