The telescopic sight would be expected for deer hunting and the reports I saw said it was a deer hunting rifle. A Mauser 98, which isn’t an assault rifle.
I think you’re getting fooled by the huge difference in hunting culture between the UK and the US; the US hunter is usually blue collar/middle class, is an experienced rifle shot and has a big freezer. The chatter I saw when the young man was identified was along the lines of ‘yes, an experienced hunter could have easily made that shot with standard hunting gear.’
This is a crime which could have equally well happened in the UK (and has). Someone with a legitimate, legal use for a gun, who then used it to murder people.