I will never understand why someone will kill someone then go and kill themselves (if that is the case). Just kill yourself first and leave the innocent to live their lives.
People often murder impulsively, out of white hot rage. It's not as if most murderers have a good plan. But anyhow, people who murder come from all walks of life, don't usually plan or premeditate for days, are ultimately triggered in some way often by something insignificant. Strangulation is the most common form of domestic homicide. Often, murderers do not regard their victims as "innocent" but mostly, they aren't thinking much about consequences at the time and live to regret it.
One man I interviewed about the attempted murder of his wife said that she had forgotten to mail a couple of bills, when he had explicitly told her to do so that day, so when he came home and saw the envelopes still on the stand in the entry way, he slapped her when she came to greet him and then tried to strangle her because she was crying and screaming, which he didn't like. She refused to testify against him in court. He's still working at the same job he had.
We aren't to discuss mental illness, so I'll just say that some murderers have longterm mental issues, others do not - but all are in an unusual cognitive state right before, during and after the murder. Serial rapists are the same. They will often say "I don't know," even years later, when asked why they did it - or why they did it the way they did it. Same is true of many unsuccessful suicides. Why on that day? Why that method? They'll say they don't know - and they truly look as if they are perplexed by their own behavior.
Kind of like asking about some more ordinary thing we might do (why did I choose these particular clothes to wear today? I don't really know exactly...)