If an accident, likely its the result of a remote starter accidentally triggered in the early morning. If the garage door is drafty and there was a central unit circulating air it could easily get to a point to overcome those in the house almost at the same time.
Or it could be a very well planned murder.
The odd thing is the guy in the shower. I would think that he would have been feeling the effects and either laid down or try to get out. Shower seems odd.