Hatred between fellow Jews, tears the fabric what holds us together. It is not just one Jew failing, it's betrayal of a deep bond.
History shows that when Jews turn on each other, it weakens the whole, leaving us vulnerable to external threats, that don't care about internal squabbles.
Think of the infighting before the distruction of the second Beis HaMikdash, the division it created was a disaster. Unity isn't some fluffy idea, it's survival.
Beyond that, hating someone close is poision for your own soul. Turning differences into chasms, and blinds you to the humanity you share.
Judaism itself pushes against this, look at the mitzvah " Love your fellow Jew " ( Vayikra 19 : 18 ). This is not optional.
When you hate your fellow Jew, you are not only hurting them, you are rejecting a piece of your own story. And in the world that is rarely kind to us, why give our enemies the satisfaction of doing their work for them.