According to Scripture, we cannot claim to love the God of the Bible while possessing contempt and ill will in our hearts for other people. It is not possible for us to know, love, and serve Him while holding animosity toward other people. We have even been admonished by the Apostle Paul to not let the sun set on our anger, as remaining in such a spiritual condition can have ominous consequences (Ephesians 4:26).
This unnatural state of hatred that the Apostle John makes mention of is utterly incompatible with the self-sacrificing love demonstrated through Jesus Christ making atonement for our sins. This unnatural state of hatred that the Apostle John makes mention of is irreconcilable with Christian charity as a whole. Thus, the people who claim to be serving God faithfully who at the same time are continually holding resentment for or bearing ill will for others in their hearts are liars. They are not acting in accordance with God's will.
Love of God and love of neighbor fulfills the Law. Love of God and love of neighbor is the foundation for morality. We cannot grow in our relationship with Him if we do not love our neighbors.
If we do not love other people as we love ourselves, then we do not really understand what love is. And if we do not correctly grasp the concept of love, then we cannot have fellowship with God because He is love. In truth, those who allow hatred to rule over their lives are acting carnally. It is one thing to have knowledge of God, but it is quite another to actually be reconciled to Him. It would be wise for professing Christians to examine their conduct and doctrine closely to ensure that they have a genuine relationship with Jesus Christ.