Oh, come on, the weird thing is that they were floating the suicide theory almost immediately.
The bizarre context behind all this is that if they had conducted any vicitimology it would have all pointed back to two violent, out of control people, in Rebecca's life with one of them basically stalking her (not to any depression, guilt, etc). That doesn't exactly lead to a strong motive for suicide, but murder.
Nonetheless, RZ's was investigated as a homicide and the ME concluded it was a suicide. Apparently, RZ never reported two violent, out of control people to police or told anyone she was afraid for her life.
The suicide conclusion does fit with the circumstances and I believe the trigger was little Max's impending death.
JMO