Since there are no details concerning what is claimed to have been tolerated from his wife, the speculation can go the other way.
What must she have been through to figure that being dead was better off than being married to him?
The fact that he pulled a shotgun on her already tells me where most of the blame for that relationship goes. No real man worth his salt would do that. If the marriage was in that kind of shape that it would push him to take that kind of action, why did he let it go so long and why didn't he try to figure out what to do to either salvage it or end it.
Cold blooded murder is not the answer.
In Islam, murdering the wife isn’t prohibited.