How are Silas's reactions realistic when he found out that something terribe had happened to him?
When Silas realized that his gold was gone and that someone had basically welcomed himself into his house for the purpose of stealing from him, his reaction was quite realistic, and very similar to the reaction that could have happened to anyone who would have gone through the same situation.
Basically Silas was not only robbed of his gold, but of his present, his future, his dignity, and his security. He was no less violated than anyone who gets their identity robbed, or someone whose luggage gets stolen from the airport, or someone who comes one day from work to make dinner for the family only to see that the things they worked so hard for are gone just because of someone else's selfishness and malice.
In Silas's case, he was also an older man and had a harder time connecting the malice of humanity to the peace that comes with isolation. When Silas began to scream and fell into a fit of rage that was quickly followed by a state of shock and grief, he was going through all the seven steps of healing which include: Denial, negotiation, anger, grief, depression, and acceptance.
Therefore, Silas's reactions were no different than those of a person in real life. In fact, they were the same as we all have to go through the seven stages in order to be able to somehow forgive and somewhat forget.