I have no idea if this was a domestic dispute gone really bad or a tragic mistake, but either way if there was no gun in the place there most likely wouldn't be a dead person.
I'm sure most of us have experienced a couple of bouts of extreme rage in our lives, rage which overrides our good sense completely. Drugs and alcohol can play a role here, of course. Those events don't always lead to violence, but they can. Some people internalize their rage, some lash out. Even when they do lead to violence, most of us just aren't capable of killing someone absent access to a deadly weapon.
Even if this was a tragic mistake, you really shouldn't be shooting at an "intruder" unless you have good reason to believe your life is being threatened.