If he’s not a Muslim convert, then this is really a “man bites dog” kind of story.
There is absolutely zero indication that he is a Muslim - none of the people who know him indicated that he might be one, including colleagues from work. There was no classic Allah crap before/during the attack.
This really is just a loner whacko who snapped.
Well, not quite as simple as that. Far down in the article you find the Mayor in the village where Simon S grew up and where the family still lives, saying: "[D]ie Familie sei bestens integriert." (The family is fully integrated.) Now this can mean that it is a family of Swiss stock that takes part in the local communal activities, like the local fire brigade etc. However, I asked a Swiss colleague and he said that it is very rare you would write that about a Swiss family. Much more likely that you would say that about an immigrant family. But then again Schwyz is one of the original cantons, so maybe the mayor just wanted to indicate that they originally came from Zürich or some other new canton.
Sad story, in any case.