I'm so sorry for your cousin, dzneprincess.
But I'm afraid I DO understand.
It is among the most common of homicides. Your words about how can you do this to someone you once loved, well, love and hate are very similar emotions. You don't kill someone you just just don't like or are indifferent about.
As sad as it is, it happens everyday. This is why the closest person is checked whenever someone is murdered. Scott Peterson and your cousin's husband will burn for what they did, whether they have deluded themselves to believe that they're not guilty (OJ, Robert Blake???) or not.
God bless you, honey,
Robinrs