Was Albert Finney's murder of PSH a mercy killing?
The first time I watched this film, I assumed the final murder to be an act of revenge. But upon a second viewing, I'm starting to reconsider that assumption.
Hoffman's character had just murdered 3 people (on top of his involvement in the initial robbery), and at the very least, would be spending the rest of his life in jail...on top of all the guilt about what he's done.
Maybe, just maybe, Finney's character believed the merciful thing to do was to kill his son, sparing him a lifetime of the agony that Finney was experiencing and which he "knew" his son was experiencing (remember the conversation in which Finney slaps Hoffman?). He also didn't seem angry when he called out to his other son, which lends water to this theory.
Or maybe he just wanted revenge. That's the genius of the film, both possibilities "work" when broken down and analysed.
Something to think about, in any case.