He didn't "have to", but absolutely could and there's several situations where refs do blow (where they want to highlight that play should be restarted). That Arsenal's player was "confused" is entirely irrelevant. Also, it doesn't particularly look like he's confused tbh, looks more like he just he'd decided that he was gonna take the kick and just picked it up.
Also, it "affecting the game" or spoiling it is ridiculous. If an Arsenal player scores a ludicrous own goal in an unopposed situation, would you just decide to discard it cos it's a schoolboy's error? Where do you draw the line? It's a CL quarter final, there shouldn't be schoolboy errors, and if they are, they should be punished.