The OP describes the story from the perspective of the deceased's family. But, they have motivation to try to make him look better, so their story needs more corroboration, like from phone records of the wife, bank statements, in other words, facts.
Nothing in their story explains why the deceased showed up at the Swiss man's house with a gun.
If it is true that the wife called Sathian before he came over, then there can only be 2 explanations:
Murder
Or
Scare the Farang away, so Sathian can move in.