The fact that you do 612 out of 613 mitsvos, or even 1 out of 613, does not exempt you from the rest. The you fail to do one mitsva is not a heter for skipping any of the others. In our tradition, you are rewarded for what you do, and punished for those you don’t. This is unlike some of our neighbors who hold that there is a weighing and you are either “saved” or “dammed”.
For example, even if you are actively engaged in murder, theft, adultery and violating Shabbos – you are still required to honor your parents (or for that matter, sending away a mommy-bird before grabbing the baby – traditionally considered the easiest mitzva in the Torah).