(א) כי ברע הוא. בְּדֶרֶךְ רַע הֵם הוֹלְכִין תָּמִיד, וּבְנִסְיוֹנוֹת לִפְנֵי הַמָּקוֹם:
(1) כי ברע הוא [THOU KNOWEST THE PEOPLE] THAT THEY ARE SET ON EVIL — They always go on the wrong way and with temptations, before the Omnipresent.
(א) הם הולכין תמיד כו'. דק"ל למה לא כתיב כי "רע" הוא אם הדבר שב לישראל: ל"פ ברע * קאי על הדרך הרע ומשום הכי הוצרך להוסיף ולומר הם הולכים כיון דבדרך נופל לשון הליכה.
ואמר "תמיד" מפני שבזולת זה הי' משמע שאמר בעבור הדרך הזה שכוונתם בו לרעה וזה א"א שאם הי' יודע משה זה הי' לו לגדור גדר מתחילה טרם שיעשו הרעה לפיכך הוצרך להוסיף מלת "תמיד" כלומר בכל מעשיהם הם תמיד ברע:
(1) They are always going. . . Rashi is answering the question: Why is it not written it say כי רע הוא , [instead of ברע ], if it refers to the Bnei Yisrael? He explains that ברע refers to the evil way. And Rashi adds the words “they are going” because “going” is the term that is associated with “way.” And Rashi adds the word “always,” because otherwise Aharon seems to be speaking of this [specific] way on which they intend evil. But this cannot be, because if Moshe had known of this [as the verse says, “You know,”] he should have taken preventative measures at the onset, before they did evil. Therefore Rashi adds “always,” to convey that in all their deeds they are always on an evil way.
* See the Avi Ezri on this passuk who also makes this diyuk that lines up with the Sifsei Chachamim's diyuk from "tamid", that Aharon was telling Moshe that the sin came from the Erev Rav who are enveloped in evil, unlike the Jews from birth.
In other words Rashi is telling us, that the Erev Rav were inherently crooked, and when someone is inherently like that, there is no way of putting up a boundary to help them; their entire way is crooked.
I'd like to bring a proof to the Sifsei Chachamim's pshat in Rashi that the Erev Rav were up to no good from the beginning. The worship of the cow/calf was a practice that was very common in the Egyptian culture. (See G-d versus gods pp. 97-98)
In footnote 93 (ibid) the author brings from the Asara Maamaros that the Erev Rav mostly married into the tribe of Shimon since, based on the Zohar, the tribe of Shimon represented the sign of Taurus, and the Erev Rav were followers of Taurus, the bull.
We see that right from the start, the Erev Rav were deliberate in their spousal choice to be similar to their Avoda Zara, and this is what Aharon was telling Moshe that the Erev Rav have been always on the evil path from the beginning.
In footnote 93 (ibid) the author brings from the Asara Maamaros that the Erev Rav mostly married into the tribe of Shimon since, based on the Zohar, the tribe of Shimon represented the sign of Taurus, and the Erev Rav were followers of Taurus, the bull.
We see that right from the start, the Erev Rav were deliberate in their spousal choice to be similar to their Avoda Zara, and this is what Aharon was telling Moshe that the Erev Rav have been always on the evil path from the beginning.