I am not arguing that murder is OK. But I do suspect that Paul Le Roux is being railroaded. I also suspect that Barack Obama, who clearly has far more blood on his hands, will never face trial.
Use the Bible. That's something a third of the world agrees on, and the whole west is founded upon it.
But seriously, murder being bad is pretty much a human moral universal. You'll find that basic idea wherever and whenever you look (+/- various caveats).