If you want some back story on Seven, try this.
After Voyager, Seven was with Chakotay, but since they were no longer trapped on a ship with a mission to get home, Seven quickly came to the realization that Chakotay is boring as fuck, so she left him. She then started sleeping around with Tom Paris behind B'elanna's back. B'elanna however started to get suspicious and confronted Seven. Seven told her the truth which surprised B'elanna and caused her to break down and cry. She confessed to Seven that her relationship with Tom was no longer what it used to be and that he wasn't satisfying her anymore. Seven agreed that she also wasn't quite getting the satisfaction she was looking for from Tom. The two made a connection. B'elanna slowly stretched our her arm and reached out to hold Seven's hand. From then on, Seven proceeded to have intimate relations with both Tom and B'elanna, unbeknownst to Tom. It went on for a while but the arrangements came to an end when Tom and B'elanna and both of their children were killed in a tragic traffic accident. At the funeral, Seven ran into Janeway, who invited her over for a drink at her place. One drink lead to another and before she realized it, Seven was involved in a romantic relationship with Janeway. She fell in love with her head over heels. One evening, when Seven came over to Janeway's place, she noticed a man coming out of the apartment. She hid behind a corner and overheard Janeway talking with the man. She was shocked to hear that they were talking about her. Janeway explained that she was just using Seven for sex and that she enjoyed her Borg stamina, but didn't actually feel anything for her. Tears appeared in Seven's eyes. The man applauded Janeway for her cunning ploy, admitting that he had done the same in the past. Then both of them laughed. Janeway and the man passionately kissed, after which they discussed meeting up again the next day. As the man left, she was surprised to learn that the man involved with Janeway, was Chakotay. Seven went into a mode of extreme rage, the likes of which she never experienced before. Long story short, she killed both Chakotay and Janeway. Then, realizing there was nothing left for her on planet earth, she stole a spaceship and embarked on a trip of exploration. She did whatever she wanted whenever she wanted it, regardless of any consequences to others. Initially, it was a breath of fresh air and Seven finally felt truly free for the first time in her life. After a while though, she found the experience to be a hollow one. One day, while sitting at a bar frequented by galactic criminals, a young girl came rushing in frantically and tried to hide. She was followed by a trio of fierce looking aliens, heavily armed. They found the girl and started beating her senseless. Seven couldn't stand the sight and intervened, mercilessly and efficiently slaughtering the three brutes with ease. The girl embraced her and thanked her with tears in her eyes. The experience gave Seven both a rush and a new sense of purpose. She finished her ale, got on her space ship, and flew around looking for people in need to assist, which she has been doing ever since.