Here John advises us that loving one another is not an option or a suggestion, it is a commandment. John doesn't want any believer to become like Adam and Eve's firstborn, Cain. That happened because Cain was under the influence of Satan. This is, therefore, a warning not to mess with the world. So, what if they hate us? We are bound for everlasting life in the presence of God. That's why death has no power over us. And that's why, if our thoughts do not tell us that we are guilty, we will not be afraid to come to God. We can pray to God and we can ask Him to help us. He will give us what we ask Him for. That is because we obey His commands and we do the things that please Him.
Furthermore, we should all love each other because God makes us able to love other people. Everyone who loves other people confirms our claim to be a child of God. If anyone says clearly that Jesus is the Son of God, God lives in that person. And that person lives together with God. We know the kind of love that God has for us. That's why if we love God, we will not be afraid of Him. So, how do we know that we love God's children? We know it when we love God and we do what He tells us to do.