This sounds like a homework assignment, so you will need to pick out the events yourself. You highlighted the word events in your question, which makes me think that your teacher wants you to focus on action, things that happen in the book, not just words.
Though I will not answer this question for you, I will point you in the right direction. Dallas is only in about a third of the scenes of the book, so you should look for your answers there. In chapter 2, he and Johnny and Ponyboy sneak into a movie theater. Why does Dallas want to sneak in? What does he do when Johnny, the only boy he really cares about, tells him off?
In chapter 9, Dallas breaks out of the hospital where he is supposed to stay in order to join a fight. I would argue that his joining of the fight is not a bitter action because he does it for the sake of Johnny, who is dying. However, the way Dallas leaves the hospital, and the way he gets back in to see Johnny after do demonstrate Dallas's coldness. How does he try to make the doctors and nurses listen to him?
Dallas's crowning act of bitterness occurs in chapter 10, when he is grieving about Johnny's death. What does Dallas do then?