He didn't mean to shoot, he was just "twitchy," had his finger on the trigger when he shouldn't have... and twitched at the wrong moment.
Yes, of course, but he was also in full panic mode brought on by paranoia based on what was happening around him.
TBH, I think that was just Rudy being incredibly stupid, and with abysmal trigger-discipline.
Not good for a police officer in the best of times, but that moment was hardly normal, and given the context in which the event happened and Rudy's reactions, he would of shot off his own big toe if it twitched because of his state of mind
Even when I do not think Rudy is random evil in disguise, I have not discarded the idea that he could have been under some kind of magical influence, like an entropy curse or something like that. I can see JB creating a sort of reverse Felix Felicis, just to have some fun with the fans of that other wizard called Harry.
I tend to agree with you, though I think Rudy suffers from paranoia. Paranoid people see threats everywhere, many not in the least rational. The kicker is good luck with trying to convince them that what they fear isn't a threat. In a normal world Rudy may have been able to continue and function pretty well as a police officer, but Harry's world isn't normal, he totally cracked and Murphy paid the price with her life. The future question will be I think is whether or not his fellow officers or the court can spot his paranoia, believe me it isn't as easy as you might think it is.