Well, it could be centered around the idea of emotional turmoil. In many novels, when a character expresses such a strong negative sentiment, it often reflects on the overall state of the relationship in the story. Maybe it's about two characters who have had a long - standing feud or some sort of misunderstanding that has led to this extreme feeling.
I think the main theme could be related to the limits of love and the power of negative emotions. Sometimes in relationships, love can turn into something very different when faced with challenges. In this novel, the statement 'There were times I wish you were dead' might signify that the relationship has reached a point where all the positive aspects have been overshadowed by problems, and the character is expressing the depth of their distress in a very extreme way.