You are correct in stating that we don't know if Terri was on meds June 4th.
It is sad that anyone is depressed, but in Terri's case, it doesn't provide her with an excuse to commit a homicide. Over NINE percent of the US population is depressed at any time. This equals 28 million people in the USA that are clinically depressed. Being depressed is clearly not an excuse for killing a child. Not being on antidepressants is also not an excuse for killing a child.
IMO, Kaine is experiencing grief and blaming himself. He has grasped on to the fact that 1 1/2 years ago, Terri was depressed. He is "beating himself up" with this thinking that somehow he could have prevented Ky's disappearance. Many people have illogical guilt while grieving. In his grieving, sleep-deprived state, Kaine's statement does not prove that Terri had severe depression (1 1/2 years ago). It does prove that Kaine is feeling guilty and this frequently happens to parents who have lost a child. It is illogical, but the feeling of guilty occurs because parents falsely believe they should be able to protect their children from anything and everything.