Why did the NT writers introduce such confusion? Is it because the NT writers were themselves not in agreement about the doctrine of the Nature of God? Or were they in agreement, but finding it hard to reconcile with the OT?
Or the early Christians drew on a variety of different OT and contemporary Jewish models of the eschatological divine agent (e.g. as a Law-giver prophet in the mold of Moses or Elisha, a priestly messiah in the mold of Melchizedek, a Davidic king enthroned in the eschatological kingdom, a Danielic and Enochic "Son of Man" figure who brings eschatological judgment, a sapiential characterization of the agent as Wisdom or Logos, the kurios and/or the Suffering Servant of Deutero-Isaiah, etc.), which led to different characterizations of Jesus in early texts and these texts were later collected into a corpus to be used and (re)-interpreted as the basis of a harmonistic theology or as the means of disproving or validating an existing theology. In the first century, the overriding concern in christology was Jesus' eschatological role and function, not on how the Son and the Father were precisely related to each other, etc. which was more of a second and third century concern.
The early attempts to define Jesus embraced both strictly monotheistic and ditheistic trends current in Judaism (cf. Metatron who eventually becomes a "lesser YHWH" in 3 Enoch, the hypostasization of the bat qol, the "two powers in heaven" interpretation of Daniel 7, the deuteros theos of Philo Judaeus, etc.), and the use of henotheistic OT texts to refer to the Son (such as the plural in Genesis 1) contributed also to a pluralistic view of God. The theological debates of the second and third centuries were partly aimed at resolving these conflicting impulses (i.e. seeing God as "one", and recognizing Jesus as "God" yet distinct from the Father), modalism representing one solution and trinitarianism representing another. In modalism, the distinction of the Father and Son is sacrificed in favor of oneness. In various theories of trinitarianism and binitarianism, the distinction between the two is maintained while the oneness of the two is also maintained without division. In later Arianism, the distinction between the Father and Son is maintained by sacrificing the oneness between the two. All of these represent different solutions of working through the material inherited from the OT and NT, utilizing newer tools such as technical vocabulary and philosophical concepts (which necessarily reinterpreted biblical texts in novel ways).
The jump between the OT and the NT only seems striking when you omit intertestamental Jewish texts that already anticipate many distinctively Christian ideas, such as the "Book of Parables" of 1 Enoch, Wisdom, Philo Judaeus, 11QMelch, etc.