The worm has turned

I have read previous posts about "the worm has turned" but they don't make sense to me. They suggest that even a lowly earthworm will attack when stepped on, which is clearly untrue.

It occurs to me that "worm" archaically meant "snake" which makes more sense. Why would Shakespeare say "even the smallest [earth]worm" when the size would hardly matter? "Even the smallest snake" makes more sense.

But when Shakespeare says "The smallest worm will turn, being trodden on" he can't have meant "snake", because this passage is a list of what various animals will do, and he has mentioned "serpent" in the previous line! (VSD)

It is certainly true that an archaic meaning of "worm" is snake; so is dragon. I'm not completely sure, but I think in Wagner's Ring, one of the dragons (Fasolt and Fafner) is denoted as a "Wurm."

The treatment of the phrase in our archive includes mentions of the earlier form of the proverb, namely, "tread on a worm and it will turn." The OED glosses this as "i.e., even the humblest will resent extreme ill-treatment." Variants of this phrase occur even before Shakespeare; the OED cites examples from 1546 and 1548.

The meaning of "turn" has aroused some speculation. Surely a mere worm turning around because you stepped on his tail is not remarkable? Here's some of what the OED says s.v. turn.

"†d. To turn in opposition or defiance; ..., to recoil on. . . ." The examples cited are from c1330 onward, and include: "1548 HALL Chron., Edw. IV 199 What worme is touched, and will not once turne again? 1641 in Verney Mem. I. 199 A worme will turne agayne if it be trod on."

In the variant, "The worm turns," often preceded by "So," or "Ah," the worm, a metaphor for someone conspicuously downtrodden or humble, recoils on his tormentor or oppressor, bites back, so to speak.
SS

I always assumed it had to do with the metamorphic change that occured when a catepillar became a butterfly. This is not exactly meant as a negative, but a positive change?