score:20
There are two differences here:
The first difference reflects an alternative translation choice for the Greek word "πονηροῦ". This might be in the masculine or the neuter gender - the word forms are the same. But there is a difference in meaning: if it's masculine, then it's referring to a specific individual who is evil, "the evil one", but if it's neuter, then it means non-personified "evil" in general. In the absence of any immediate grammatical clue, it's a matter of judgement which reading to choose. The NIV translators simply made a different call from those who translated the KJV.
The second difference reflects the fact that not all early source texts include this bit of the Lord's Prayer. In fact, there is a denominational distinction as well: the standard Catholic version of the Lord's Prayer does not include it, but most Protestants do put it in. It is not present in the oldest manuscripts of Matthew, but there are versions that include it, so again it is a matter of judgement between the two sets of translators as to which text version is "better".
Upvote:3
The reason for the presence or absence of the brief doxology at the end of the Lord's Prayer is actually liturgical use.
In Eastern-rite usage, the doxology is recited in the liturgy after the Lord's Prayer; this is probably how the phrase crept into Eastern Greek mediaeval manuscripts of the New Testament. These manuscripts are ultimately the ones on which the KJV was based. The KJV served as the Bible for English liturgy for a long time, thus becoming traditional for Protestants.
On the other hand, the doxology never made its way into the text of the Latin Bible, which was the basis of Western-rite (Roman Catholic) liturgy. So it is not understood as part of the Lord's Prayer proper in the Catholic Church.
Newer Protestant translations make use of a scholarly critical text of the New Testament. Biblical scholars stand in general consensus that the original Gospel of Matthew did not have the doxology. They believe this because the earlier Greek manuscripts don't have the phrase, and neither do early translations of the text, while there is a clear motivation for adding it in later Greek manuscripts. So the phrase is absent from the text of most modern translations (though it might be noted in a footnote).