Does the warning of entry into Hellfire for those who quarrel apply to both the oppressor and the oppressed, or only to the aggressor, especially when avoiding the aggressive quarreler is better to avoid further problems?

There is no Hadith stating that one who harbors animosity (al-mutashahin) will enter Hellfire. However, animosity prevents forgiveness. On the authority of Abu Hurayrah, the Prophet (peace be upon him) said: "The gates of Paradise are opened on Monday and Thursday, and every servant who does not associate anything with Allah is forgiven, except for a man between whom and his brother there is animosity. It will be said: 'Wait for these two until they reconcile.'" And on the authority of Abu Musa Al-Ash'ari, he (peace be upon him) said: "Indeed, Allah looks down on the night of mid-Sha'ban and forgives all His creation, except for a polytheist or one who harbors animosity." Animosity here means enmity, and it includes those who harbor animosity towards each other. However, if the oppressor is the only one at fault and the other person does not reciprocate the enmity, then we hope that this (prevention of forgiveness) will not apply to them.

It is permissible to be reserved with a harmful person and suffice with merely greeting them, based on the Prophet's (peace be upon him) saying: "The worst of people is he whom people abandon to avoid his evil." One should counter their injustice with kindness and forgiveness, as Allah Almighty says: "Repel [evil] with that which is better, and thereupon the one whom between you and him is enmity will become as though he was a devoted friend" (Quran 41:34), and His saying: "So whoever pardons and makes reconciliation - his reward is due from Allah" (Quran 42:40).

As for deliberately boycotting Muslims, it is forbidden, based on the Prophet's (peace be upon him) saying: "Do not envy one another, do not turn away from one another, and do not boycott one another," and "It is not lawful for a Muslim to abandon his brother for more than three nights," unless mingling brings harm in religious or worldly matters.

The Prophet (peace be upon him) accepted the Islam of Wahshi, but he did not like to look at his face as a reminder of his killing of Hamza, and Islam obliterates what came before it.