You need to write balanced characters to resemble life and be believable. Some people might be dominated by anger, but have moments of compassion and understanding. Your characters should possess various traits to keep your work from becoming tedious. Keep your readers guessing how the figure is going to react in each scene. Is he in a good mood or bad? Is an argument ensuing or understanding?
Conflicts make interesting writing. Have each conflict stem from different catalysts and end in different outcomes. Variety balances your personalities. Your main character might be hard to work with and most coworkers can’t stand him but have one person who likes and supports him. The overall hatred is balanced by the one person, even though the reader sees the character as despised by everyone.
Bad habits are really personal conflicts that don’t have to be spelled out as such. All of us struggle with idiosyncrasies we would like to change. If the character bites his nails, he certainly can’t do it in every scene. And you can change up how he does it; in one scene, “He bites the nails”. In another, “…. as his right-hand moves towards his mouth.” or “He struggles to keep his hand in his pocket.” conveys the message, because we already know he’s a nail-biter. Rephrase to balance mannerisms.
In another example, “The serial killer hates people, all people, though he loves his cat”. This shows a relatable human side. An outlet for love that we all carry. He might have moments when he tortures the cat, but the overall need for love comes alive. The conflict between love and hate is clear.
Write relating to yourself or people you know, as you perceive them.