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In that situation, without context, you don't even know who the aggressor is/was. He could be pointing the gun at a burglar in self defense for all we know.
Hence why common sense needs to be applied. If you just arrest everyone who points a gun and convict them on the grounds that they may have used it to commit murder, then that would clearly be absurd.
Anyway, I'll put myself in the "the law is an ass" camp, and be done with it Otherwise we could be here all day
You wouldn't necessarily arrest the person with the gun, but you would investigate the situation. If, in the course of that investigation, the person commited another crime, you'd then arrest them for that.
That is what happened in this case. The police found a man, drunk, in a car, with the keys in the ignition. While investigating this, they asked for a breath sample (common practice if a person is suspected of being drunk). The man then commited a crime by refusing to give a sample of breath, for which he was arrested and charged.