Etymonline says:
surefire (adj.) also sure-fire, by 1864, American English, from sure + fire (v.). Originally of rifles.
Since early rifles were rather less than completely reliable, a "sure-fire" rifle would be highly regarded.
There is no hard-and-fast rule on capitalizing the word "nature" as it is not classified as a proper noun. The reason we capitalize the proper noun is to differentiate it from other common (non-proper) nouns as it is
a noun that in its primary application refers to a unique entity, such
as London, Jupiter, Sarah, or Microsoft, as distinguished from a
common noun, which usually refers to a class of entities (city,
planet, person, corporation), or non-unique instances of a specific
class (a city, another planet, these persons, our corporation).
When you capitalize such words as "god" and "lord", you no longer think about their common attributes or characteristics, but you conjure up only one entity in your mind called God and Lord.
According to the linked Wikipedia article about capitalization:
Many European languages traditionally capitalize nouns and pronouns
used to refer to God, including references to Jesus Christ
(reverential capitals): hallowed be Thy name, look what He has done.
Some English authors capitalize any word referring to God: the Lamb,
the Almighty; some capitalize "Thy Name". These practices have become
much less common in English in the 20th and 21st centuries.
If you think Nature is as unique as other proper nouns and it was created by God, there is no reason that you can't capitalize the word nature. However, when you are referring to *the basic or inherent features, character, or qualities of something", you should not capitalize it. You could only capitalize it when it means
The phenomena of the physical world collectively, including plants,
animals, the landscape, and other features and products of the earth,
as opposed to humans or human creations
It will largely depend on your personal style and preference whether to capitalize "nature" or not. You capitalize it only to emphasize its uniqueness.
[Wikipedia, Oxford Online Dictionary]
Best Answer
First, apart from very rare exceptions, you should capitalize all words derived from a person’s name (see here and there, on this very site). It doesn't matter whether it's a noun, an adjective, a verb, anything. Just put a damn capital!
Now, for your specific case, two additional points in support of capitalization:
the New Oxford American Dictionary lists all derivatives with a capital: Machiavel, Machiavellian, Machiavellianism.
of the Corpus of Contemporary American English’s most recent 100 recorded uses of Machiavellian, only 3 use the lowercase.