Guarding the hatred of Amalek as protection from sin

כתיב אשר שלט האדם באדם לרע לו.

It is written, "when one man rules over another to his own detriment" (Koheles 8:9).

The Sefas Emes opens with Koheles' phrase about one man dominating another "to his own harm," which he will apply to Amalek's power over Yisrael.

עיין זוה"ק פ' משפטים.

See the Zohar HaKadosh, Parshas Mishpatim.

He directs the reader to the Zohar in Mishpatim, the source for the idea he is about to develop.

ומה שיש לעמלק כח להחטיא את ישראל הכל לרעתו שעי"ז יבוא עלי' הקנאה ונקמה מהשי"ת עד שיהי' לעדי אובד.

And the very power Amalek has to cause Yisrael to sin is entirely to its own detriment, for through it the jealousy and vengeance of Hashem will come upon it, until it is utterly destroyed.

This is the "to his own detriment": Amalek's ability to trip up Yisrael is in fact its own undoing, since by harming Bnei Yisrael it draws down Hashem's zealous vengeance, which will eventually wipe it out completely.

אך אם בנ"י שומרין את השנאה כראוי א"צ להיות חוטאים.

However, if Bnei Yisrael guard the hatred [of Amalek] as is fitting, they need not come to sin.

The safeguard is in Yisrael's own hands: if they properly keep alive the commanded hatred of Amalek and all it represents, they will not fall prey to its enticements to sin.

וז"ש רגזו ואל תחטאו.

And this is the meaning of "Tremble [with anger] and do not sin" (Tehillim 4:5).

He reads the verse as counsel: rouse the proper indignation against Amalek and evil, and through that very anger you will be shielded from sinning.

וז"ש במדרש לגבי חומר גביכם שע"י ששוכחין שנאת עמלק צריכין לעבוד בחומר ובלבנים ע"ש:

And this is the meaning of the Midrash regarding "the mortar is against you" — that because they forgot the hatred of Amalek, they had to labor with mortar and bricks; see there.

The Midrash links the bondage of mortar and bricks to a forgetting of the hatred of Amalek: when Bnei Yisrael let go of their guard against evil, they fell into the harsh servitude — a warning that abandoning that vigilance leads straight to enslavement.

Summary: Amalek's power to make Yisrael sin is, per Koheles, "to its own detriment," for it draws Hashem's vengeance upon itself until its destruction. But the defense lies with Bnei Yisrael: by properly guarding the commanded hatred of Amalek — "tremble and do not sin" — they are protected from its enticements, whereas forgetting that hatred is what plunges them into servitude.