The theme of pity is certainly important in The Heart of the Matter. In fact, the sentence in which the title of the book first appears features pity:
If one knew, he wondered, the facts, would one have to feel pity even for the planets? If one reached what they called the heart of the matter?
Greene also discusses pity in the novel's preface. Henry Scobie, the main character, has good intentions but is "doomed" by his sense of pity. Greene mentions that human pity is not quite the same as compassion and can be disastrous to human beings.
In Scobie's case, keep in mind just where his sense of pity comes from. For example, he pities Helen and Louise. Even though he doesn't love Louise as his wife (more so as someone he pities intensely) and his affair with Helen can be considered immoral, he feels responsible for their happiness and just can't bring himself to end the relationships. This overwhelming need he feels might just be a form of pride.