As far as I now the better translation is not 'fulfil' but 'complete'. that's the clear meaning of the Greek as mean's something rather different from that which 'fulfil' means.
That said, we only have the word of an anonymous author for this word was used by Jesus. However, it certainly leaves us with the idea that Matthew may have been writing for converted Jews who valued the Torah and thus the completion allowed it to continue. For Paul, on the other hand, getting rid of the constraints of the Torah was very much his purpose when he came to Jerusalem for 'discussions' (see Acts 15). We see different factions in the earlish church and maybe translators or old wanted to make their translations consistent even when the underlying text was not.