Question: When Damian learns that he is the Antichrist, he is very upset about it. Later in the movie, why does he choose to embrace who he is and his destiny?
Question: When Damien killed Mark, he screamed and started to cry. Was he truly upset that he killed him, or was he putting on a show for his aunt and uncle?
Answer: Damien is still very conflicted, having thought of himself as a regular boy his entire life. While he is slowly embracing his nature, he hasn't lost his emotions, and he's crying because he's actually sad that he had to kill his friend.
Answer: He was truly upset, as he had not come to terms with being the Antichrist yet.
Answer: Having lived as a "normal" boy, Damien is initially shocked to learn his true identity. After having time to adjust and being surrounded and groomed by satanic supporters, he eventually embraces his true persona.
raywest ★
The answer is correct, and I'm just going a into a bit more detail. During the events of the first film, Damien is normal until Mrs Baylock entered his life. She started teaching him about who he was, but he was only five or six, which would have been far too young for him to understand. He started acting more malicious after this point. After the movie ended, it skips ahead to Damien living with his uncle, aunt, and cousin. During that time, it's conceivable that the forces surrounding him decided to give him a period of peace. He was raised by his aunt (secretly a Satanist), but she might have been instructed to give him a normal upbringing so he wouldn't call attention to himself. As a result, he probably forgot about it, and started to believe he wasn't any different. Once he learned of his unholy lineage he began to remember and eventually embrace it, especially since he had several followers (Buher, Neff, and many others he may not have met yet) to help and protect him.
dewinela