@MovinGroovinBarbie is invoking the No True Scotsman fallacy, and quite frankly I’m sick of seeing it.
She’s decided no sane person kills someone, therefore all murderers are mentally ill (or out of control, or ‘snapped’ or committed ‘a crime of passion.)
Anyone with experience of this, personally or professionally, knows this is not true. This is about control, blame, entitlement or pride, not mental illness. The articles and studies posted upthread all make this clear.
These are also frequently planned killings. The recently sentenced one where the ex murdered his former wife had already dug the hole in the woods before he drove over to kill her.
@DollyTrolly , I am so sorry for your loss. I tangentially knew a family who suffered similarly, and have nothing but contempt for those people who seek to justify and make excuses for violent, controlling, abusive men.