"Hey, pal! Can I help you? You seem distressed!" strikes me as a good option for a Christian--if he or she is a safe distance away (or can't get away). Man on Balcony didn't yet know what the kid had done, so I think it was rather unkind of him to start off with insults.
At present it looks like the Munich horror was more of a self-pitying teenager revenge attack on society than another Islamist atrocity. I have kept up to date on Islamist attacks since 2006, and Islamist terrorists tend not to throw "I was born here" hissy fits.
What an irony for a kid who strove to be identified as a German to have his actions marked down as the work of a foreign Islamic extremist. Well, whatever else we find about him, I think "stupid" will be one adjective that applies.