What is the theme of Shakespeare's sonnet 147?

The theme of evil is also present as relates to
illness/love.  The final couplet gives the source of the "patient"'s
sickness:

For
I have sworn thee fair and thought thee bright

Who art as
black as hell, as dark as
night.

These analogies allude
to the evil that is infecting the ability of the speaker to listen to his reason and, as
the saying goes, "cure thyself."  The speaker is so mesmerized by the object of his
desire that he concedes that death is what awaits
him:

Past cure
I am, now reason is past care,

My thoughts and my discourse
as madmen's are.

He describes
his love almost as a demonic possession that has driven him mad and will soon claim his
life.  But, as a madman might, he clings stubbornly to his
disease:

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...longing still

For
that which longer nurseth the disease

Feeding on that which
doth preserve the ill.

His
"desire," he concludes "is death," and since he shows no signs of recovery, of giving
his desire up, then he admits that the evil that is his love has won his life.  This
love that the speaker equates with evil is often noted to be the "Dark Lady" who figures
prominently in the later sonnets.