While I am not defending the practice, the idea of amending the text of Scripture and even inserting comments directly into the text has been going on ever since the words were first written.
Hence, for example, the term "Septuagint" (LXX) refers to a group of texts, with each carrying differences.
Not only does the expression "Masoretic Text" (MT) also refer to a group of Hebrew texts, but the LXX is based on a Hebrew version that is different to that which underlies the MT. Just compare, for examples, the LXX and MT of Jeremiah and of Daniel 9.
The early Christian Church preferred the Theodotion version over the LXX, which was likely based on a pre-Christian Greek version known as ur-Theodotionic. The Church then created its own versions, such as the Vulgate.
Even today, the Scriptures accepted by the major elements of the Christian Church (such as the Roman and Orthodox) have versions quite different to those accepted by most Protestant persuasions. Then there are arguments whether the Protestants should accept the "Textus Receptus" or the Westcott and Hort or whether a translation should pick and choose among the various sources, as they commonly do, employing disciplines such as Textual (Lower) Criticism.
So, while it is correct to point fingers at the NWT translators for the blatantly biased manipulation of the Jews' written records, take a step back and see this as a common practice.
Doug