Chad Gadya is an Aramaic song sung at the end of the seder. Yet many of the pronunciations of the verbs are incorrect for Aramaic. This is liekely due to the fact that most people don't speak Aramaic.
examples: DiZabin Abba BiTrei Zuzei.
Zabein = sold. Zevein = bought. That father bought for two zuzim = Dizvein Abba BiTrei Zuzei
Also, VeAta. Vav Sheva Aleph Kametz Suf kametz Aleph. That would mean "and comes," present tense. We want past tense, so there should be a patach under the Vav, a chataf patach under the aleph. In general there is a tendency to impose Hebrew past tense verb forms to the Aramaic words.
One exception is the nouns, which are consistently in Aramaic. Gadya, Shunra, Kalba, Chutra, Nura, Maya, Tora.
Then, Shochet, Malach HaMavet, HaKadosh Baruch Hu.
I'm not sure about shochet (it also breaks the pattern and perhaps should be Tabecha), but the latter two are in Hebrew, not Aramaic. The definite article (the) in Hebrew is Ha, in Aramaic is kametz Aleph at the end of the word. So We would expect perhaps something along the lines of malacha demavet, or demaveta. And certainly it should be Kudsha Brich Hu, rather than HaKadosh Baruch Hu.
Three possibilities: 1) It was originally composed like this, perhaps early on for some odd reason, or late, by an author who did not know his Aramaic so well. 2) It was originally in Aramaic and for some odd reason the last few stanzas had the additional nouns in Hebrew. 3) The song originally did not have HaKadosh Baruch Hu, or the Angel of Death, and perhaps not the shochet. This is a song to keep the kids awake to the end of the seder, and was perhaps not religious in origin. Perhaps it ended with the cow, or will the man at the top of the food cycle, but was made religious with the addition of the extra stanzas.
Just ruminations. Please let me know in the comments if you can shed further light on this. Thanks.
Update: Add to that another stylistic change. Every stanza from the first part introduces a new, interesting verb. Bought, Ate, Bit, Hit, Burned, Extinguished, Drank. The last 3 stanzas are repetitive and thus unimaginative in their verb: Shechted = slaughtered.