Give some examples of nature in Act 1 of Macbeth that reflects the decline into evil.

You have done well to notice the link between nature and
the evil inherent in the play. This is a key theme of this excellent tragedy as it is in
Julius Caesar. Clearly, the most notable incident of nature itself
matching the tone of the play comes at the very beginning, with the thunderstorm in
which we are introduced to the witches. Note how this sets the tone for the rest of the
play: we are plunged immediately into a world of darkness, of primeval power and
strength as reflected by the lightning and
thunder.

However, for a far more interesting example of
what you are talking about, I would want to talk about the beginning of Act I scene 6,
when Duncan, much like a lamb to the slaughter, approaches Macbeth's castle. Note what
he says about it as he comments, rather naively, on its
beauty:

This
castle hath a pleasant seat; the air

Nimbly and sweetly
recommends itself

Unto our gentle
senses.

Note the dramatic
irony of the passage: we have just seen the Macbeths plotting to kill Duncan, and yet to
him, this castle seems to be a place of rest and peace. Now, it is important to remember
that the theme of appearance vs. reality is incredibly important in this play. In the
previous scene, we have just scene Lady Macbeth counsel her husband to "look like
th'innocent flower, / But be the serpent under't." Characters who appear to be good on
the surface are actually evil beneath, and this is an excellent example of where nature
itself conspires with Macbeth and his wife to deceive the King and lead him in to his
doom.