4 ideas
Tracing back, the novel reveals Margio's father Komar beat his mother Nuraeni for years, Nuraeni had an affair with Anwar Sadat and bore a baby who died soon after, and Komar died before Margio could take revenge on him. The final trigger is Anwar Sadat's refusal to take responsibility for Nuraeni.
The narrative states who killed whom and how in the opening pages, then withholds only the reason until the end. This moves the reader's question from 'who did it' to 'what made this inevitable', so suspense runs on causation instead of identity. By the time the motive arrives, the reader has spent the book inside the conditions that produced it, which makes simple condemnation harder.
The white tiger passed down from Margio's grandfather can be read literally as village magic or as a local idiom for rage that forms across generations and then takes over a person. Framing violence as a possessing creature both explains it and removes the killer's authorship of it. That lets a community account for an act that ordinary moral language cannot hold.
Rage built up by watching a parent be abused does not go away when its proper target escapes. When Komar dies before Margio can confront him, the violence waits and then lands on the nearest man implicated in his mother's suffering. The person who pays is the one who is still within reach, not necessarily the one most to blame.