Sounds to me like it was an accidental shooting. Apparently they were having marital, and financial problems, that, I would bet he's downplaying. Says here,

"Kirk denied accusations that escalating marital conflicts and financial struggles could have been motives for murder..."

...and here

"Twelve minutes into the call, the detective said she panicked and told the operator “I don’t know what my husband is going to do.”

She says “‘my husband had marijuana.’ And then you hear her scream, ‘Don’t go in there! Stay away from the gun! Stop! Stop!'”
By that point police still had not arrived. Wisgaard said Kristine Kirk’s screaming and running could be heard followed by the fatal gunshot."

Lots of stress CAN cause all sorts of psychological issues. My guess is he was attempting to treat this stress with the edibles, of which he had a very small amount, had a stress induced psychotic break, he went to get his gun to harm himself, there was a struggle over the gun when his wife tried to stop him, and he accidentally shot her in the struggle. Him being in the state of mind he was, it's probably a big blur, so he's blaming the edible which had very little to do with his state of mind giving the insignificant amount he had in his system.