I have to wonder if this is part of a broader impact of the patriarchy…
The dude surprised his wife and her lover (a man) doing “it”. He goes into another room, gets a gun and kills the dude.
One way to think about this situation is that the lover was violating the man’s property (i.e. the wife), thus — killing the lover is justified.
It could be that they believed the idea that the man thought his wife was being raped… not so sure about this.