The dog deserved it.Yes, I do agree with you that removing the dog would have made things somewhat different for him. But for how long? Until he found some other thing that was causing him distress? In my opinion, there must have other underlying reasons for his frustration.
I do understand that for him quitting might not have been an option and all sorts of wrongs might have been committed against him. Still, this does not justify his actions.
There probably wasn't any deliberate long-term planning. However, carrying a knife and then taunting the dog to gain courage to slash her with the intent of getting rid of the dog once and for all still involves a long term harbouring of anger directed at an animal who cannot know better than to bark at people.
There is always a way to relieve tension and, in my opinion, he found a bad solution. And now he's out of a job as well. Nobody wins. Has he gotten rid of his frustration? I don't think so.
Ok, this is all pure speculation and it's pointless to go on with this.
How about mere obligation to remain humane? No matter how much of their functionality or affection is involved. If you take on a responsibility, you better be responsible for it til the end. If you have no intention of being responsible about things, how about just not getting involved with them? It's that simple.