What is a Torah defined rasha? The starting point is with the verse (Leviticus 19:17), Lo tisna et achicha bilvavecha…Do not hate your brother in your heart…”
On this verse, the Ohr Hachaim notes that there are exceptions, where you can hate a fellow Jew. This is based on the verse, (Exodus 23:5), “Ki ro’eh chamor sonecha rovetz tachat masa’o…If you see the donkey of someone who hates you fallen down under its load…” The Talmud (Pesachim 113b) derives from this verse, based on the word “sonecha,” someone who you hate or who hates you, that if you see someone perform certain sins then you are permitted to hate them.
Which exact sins can you hate a person for? The halakha says (See Choshen Mishpat 34:2) that one can hate one who transgresses a Torah prohibition that has upon it a death penalty or malkut, lashes. (Your rav should be consulted for a more involved and comprehensive analysis of this topic and how it should be applied. However, this captures one basic track as to how to see a rasha.)
The challenge now becomes how to overcome that large space of where one can't practice sinat chinam, namely when the person isn’t a halakhic rasha. Though one knows the halakhic parameter, how in fact can one succeed in practicing it?
Rabbi Yaakov Harari, rosh yeshiva of Ohel Torah, gives an eitzah on how to remove hate from one's heart. If one feels any kind of unsanctioned hate towards someone, one should list and focus in on one good trait that they can find in that person. When you do this in earnest, you will come to appreciate them and have an affinity towards respecting and loving them.
There's a logical reason as well why to love your brethren. I came across it in a sefer. If one can imagine a father and ask: what brings the greatest joy to him? One answer will be when his children get along, and we are referred to as G-d’s children, as the verse says, (Psalms 103:13), Kerachem av al banim…. As a father has compassion for his children…” It’s rational to say that any child that is peaceful and close with his other siblings brings feelings of satisfaction to the father. A sibling that can't love and get along with his other siblings brings dissatisfaction and pain to the father. Therefore, seeking peace and love with all our brethren will bring inordinate feelings of love back to us from the Heavens.
The famous question is asked, that at the time when the First Temple destruction, the churban, was taking place, the Jews noticed that the keruvim were hugging each other (Yoma 54a). This seems to contradict another Gemara (Bava Batra 99a) that says when the Jews were not following G-d's will the keruvim wouldn't even face each other. How then to explain the embrace during destruction? Perhaps the answer can be that at the time of the first churban there was unity; there was no sinat chinam. For this G-d embraced us, and said I will never let such a people go.
Let us find one positive trait in our fellow person and by doing this we can, as the Zohar (Vayechi 86:2) says, awaken G-d in the heavens, based on the principle itaruta d’ltata; for the itaruta d’layla, meaning that an awakening from below, from this world, brings an awakening from Above, from G-d’s world. In this way we can be vessels of peace and the final words we say in Shemoneh Esrei can be fulfilled: “Oseh shalom bimromav hu ya'aseh shalom aleinu v'al kol yisrael v'imru: amen - May the one who makes peace on high bring peace to us and to all of Yisrael And we say: Amen.”