The Korova milkbar sold milk-plus, milk plus vellocet or synthemesc or drencrom, which is what we were drinking. This would sharpen you up and make you ready for a bit of the old ultraviolence.
There are actually two different uses going on here. I think the "would" in "This would sharpen you up" is used to transform the statement into a conditional or hypothetical. For example, "That painting would look nice over the fireplace." In this case the implied condition under which "sharpening up" will occur is the drinking the milk-plus.
In the Bundy examples, "would" is preferred because they are generalized statements about his tactics rather than a recital of his crimes. The same generalization is accomplished without the use of "would" by starting a sentence with "in many cases," although the meaning of the sentence would not significantly change had the author chosen to include the word "would".
The last two examples are perfectly valid constructions either way, but you might prefer one over the other depending on context. The meanings are only very subtly different.
The Aztecs used shells and jewels to create dyes.
The above sentence simply states a historical fact about what the Aztecs did. It works well in multiple contexts.
Example A: "I walked through the exhibit. In one diorama, the Aztecs sacrificed a human being. In another, the Aztecs used shells and jewels to create dyes."
Example B1: "Archaeological evidence suggests that the Aztecs used shells and jewels to create dyes for application to clothing, ceramics, and architecture."
Example B2: "Archaeological evidence suggests that the Aztecs would use shells and jewels to create dyes for application to clothing, ceramics, and architecture."
Note that in Example A, "would use" would not be appropriate. However, in Example B, "used" and "would use" are interchangeable without losing the essential meaning.
The Aztecs would use shells and jewels to create dyes.
The above sentence states the same historical fact, but generalizes it as a habitual aspect in past time. As an example, it might be used when describing a scene or trying build a sense of atmosphere. The "would" might also be used for emphasis or contrast.
Example C: "The Aztecs would gladly use shells and jewels to create dyes, but they would never use turmeric."
Example D: "In the fourteenth century, Aztecs would use shells and jewels to create dyes, toiling for untold hours to achieve what is now accomplished by simply boiling powdered cochineal insects. How fortunate we are to live in such an enlightened age."
Again, in the Example D above, you could substitute "used" for "would use" without greatly affecting the meaning. However, "would use" is favored by the author to call attention to the abnormality.
Edit for more examples:
Remember that "would" in this usage describes habitual, or typical behavior.
I'm going to drop the "only" from your final example, because the modifier makes the two statements roughly equivalent. There is more contrast without it.
He spoke of it when he was under a lot of pressure
This statement means he spoke of it at least one time, and he was under pressure at the time that he spoke of it.
It can be considered a discrete event, solidly in the past.
He would speak of it when he was under a lot of pressure
This statement means that whenever he was under pressure, he spoke of it. It suggests that this occurred more than once. The central meaning conveyed is not the past event(s), but the past behavior: activity that was typical, or which could be generalized. This is why we call it "habitual."
The behavior is typical, such that if you traveled back to the timeframe being discussed, the occurrence "would" happen again (conditional/future tense) if the correct conditions were supplied.
Here are a few more examples that may shed some light on the different scenarios:
Simple Past for Discrete event(s):
Last month, I stopped at a gas station for a bite to eat after work.
Seven times last month, I stopped at a gas station for a bite to eat after work.
Simple Past with a frequency for Habitual Behavior:
Last month I frequently stopped at a gas station for a bite to eat after work.
Would for Habitual Behavior:
Before getting married I would stop at a gas station for a bite to eat after work.
Would for Contrasting Habitual Behavior:
In previous years I would stop at a gas station for a bite to eat. Now I wait until I get home.
Would for Habitual Past Tense Conditional:
I would stop at a gas station for a bite to eat if it wasn't too cold outside.
And for the sake of completeness...
Would for Present Tense Conditional:
I would stop at a gas station for a bite to eat if it weren't so cold outside.
Would for Discrete Past Tense Conditional:
I would have stopped at a gas station for a bite to eat if it hadn't been so cold outside.