The conjunction “so” is conventionally used to introduce clauses of result in the sense of “with the result that,” as in this sentence: “The pipeline has been fixed, so the water is flowing now.” On the other hand, “so that” is conventionally used to indicate purpose in the sense of “in order that,” as in the second sentence you presented, “The old spiced it with terror so that the rest of humanity would observe superstitions.” But this distinction in the sense of “so” and “so that” has practically disappeared in modern usage. In my case, in fact, I wouldn’t take issue with anyone who uses “so” instead of “so that” to come up with the sentence “The old spiced it with terror so the rest of humanity would observe superstitions.” However, I’m inclined to agree with grammar purists when they insist that the use of “so that” in this sentence is iffy and rather awkward: “The pipeline has been fixed, so that the water is flowing now.” So, when the clause being introduced is that of a result, just stick to “so” and forget “so that” as an alternative. You can’t go wrong with that choice.
