Anger & Merit

Shantideva, one of the great Mahayana sages, once said:

Good works gathered in a thousand ages,
Such as deeds of generosity,
Or offerings to the Blissful Ones-
A single flash of anger shatters them.

Why does anger destroy all the merit you ever accumulated?

Because it is you!
In the same way that eons of ignorance can be banished by a single moment of enlightenment, ages of merit can similarly be destroyed by a moment of anger.
Anger is you, love is you, merit is you, sin is also you. It is still the same one mind, but use it for anger, and it becomes hell. Use it for love, and it becomes heaven. Use it for ignorance, it becomes sentient beings; use it for enlightenment, and it becomes Buddha. Whether merit or sin depends on how you use this one mind; but at the end of the day, nothing is created, nothing is destroyed. It is just you.

Don't you see?