He came up on it quickly, it was not aggressive initially, it charged and gave him plenty of warning and opportunity to retreat. To the moose the guy on the snowmobile was a predator and he did exactly what his instinct told him to do.
The guy had no justification for shooting the animal. He wasn't even a particularly bad tempered moose and probably suffered while it died.
You see similar stuff all the time on Front Range trails, people coming up on rattlesnakes and poking them with sticks trying to get them to move and then they're shocked when they get bit. You have encroached on them, you need to back off and go around. Plus they don't know that humans made the trails for recreation, they just see ideal places to walk and sun themselves.
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"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler