The Chida zt”l (Sefer Tzeil Ha’Eida) explains the severity of not fulfilling a vow by writing that, as is well known, every mitzvah that one performs creates an angel that advocates on one’s behalf in Shomayim. Therefore, when one vows to do a good deed, he begins creating a malach. However, this angel is not completed until he fulfills his promise and does the mitzvah. As long as he fails to keep his word, the malach is incomplete and blemished. Thus, the reason one who delays fulfilling a neder is punished is because of the pain he causes to the malach.
For this reason, Chazal say; “When one starts a mitzvah, we tell him to complete it.” As long as he does not finish a mitzvah that he started, his malach is still blemished and is waiting for its completion.
