Why can’t we delete the Brocho of “״ולמלשינים in Shemoneh Esrei? Wasn’t The Rebbe all about bringing Jews back and not pushing them away?
Firstly, it is not our place to “correct” and edit any part of the Siddur. This Brocho specifically was composed by Rabban Gamliel in Yavneh with approval from all of the Sages of Israel (see Brachos 28b).
Beyond the literal meaning of the Brocho, it also conveys a prayer for the times of Galus where the darkness is very strong, that is the Klipah and Sitra Achra. We ask and pray that the Klipas Nogah be subdued and the three level of impurity should all be eradicated. This is their only Tikkun (fixing).
Additionally, the hatred of evil is fundamental to Yiddishkeit. A proof of love of Hashem is when there is absolute hatred of evil.
See this idea explained at length in Tanya Perek 10.
“…The “Incompletely righteous” is he who does not hate the Sitra Achra with an absolute hatred; therefore he does not also absolutely abhor evil. And as long as the hatred and scorn of evil are not absolute, there must remain some vestige of love and pleasure in it, and the fouled garments have not entirely and absolutely been shed; therefore the evil has not actually been converted to goodness, since it still has some hold in the filthy garments, except that it is nullified because of its minute quantity and is accounted as nothing…”