This is easier to explain once you realize many nouns in Japanese can be used as adverbs (or just "verbal complements" if you prefer, I don't care how you call them). In fact, most of the Japanese words that dictionaries describe as adverbs also double as nouns or adjectival nouns (na- and no-adjectives). Take 普通, for instance: it can either be an adjectival noun meaning "(being) normal" or an adverb meaning "normally", "usually".
This allows Japanese to easily fashion new adverbial constructs out of nouns, where other languages would require the special class of words called conjunctions (which barely exists in Japanese). For instance, to say "when", we just use one of the words for "time", [時]{とき} in an adverbial role, so we get:
町に行ったとき、 The time that I went to town-ADV
新しい着物を買いました。 I bought a new kimono.
Since とき here is an adverb, it describes the verb, so you could treat it as if the preposition "at" was added before it to make its English equivalent adverbial:
"At the time I went to town, I bought a new kimono"
Now, back to ほど: It behaves in the same way. Let's take the sentence:
深ければ深いほど暗くなる。
Imagine me adding commas here to separate all three parts of the sentence:
深ければ、 When/if it's deep,
深いほど、 the extent that it's deep-ADV => to the extent that it's deep
暗くなる。 gets darker.
I've done the same trick again: I've added a preposition to "the extent" to indicate it's used adverbially (it describes how things are getting darker, and they are getting darker to the extent of it :)).
So if we look again at the last two parts, we have:
"It gets darker to the extent that it's deep."
So why do we need the conditional at the beginning? Well, there's no such thing as "must have" in a language, and if you want a simple answer, then that's just the way such expression ended up working in Japanese. But there is a perfectly valid historical reason for this pattern becoming the normal way of say "the more X the more Y". If you've said 深いほど暗くなる, you'd just imply that there's a consistent link between the extent of deepness and that of darkness.
But you'd usually want to topicalize one of the linked extents (that is, for instance, to mark it as the one the other guy was talking about, and to which you now link a second extent). For topicalization to work, you make a condition out of it, which boils down to something like this in a chatty translation.
"Well, you were talking about going deeper, right? If we go deeper, the deeper we go the darker it becomes."
I think this is the source of the conditional, but now it's pretty much the norm, and there's hardly anything else to contrast it with.
Edit:
I think I've misunderstood you quite a bit, but I hope my answer is still clear: ほど does indeed have meaning of "extent" and of "more", but when you say 深いほど暗くなる then you mean that it really gets darker to the extent it gets deeper. If you're very shallow then you'll have very little darkness, but if you get deeper, the extent of depth will be increased and so shall the darkness.
Yes, this does seem to be a slightly different meaning than in 殺したいほど嫌いだ, where the clause coming before ほど isn't perceived as something measurable but rather something that either happens or doesn't happen.
