Aloha !

I think Courtney did kill him or gave him something that made him do it. At the time it happened he was splitting from Courtney, he was about to take his daughter with him, he was off drugsHe was splitting from courtney he was taking his daughter with him. He was off drugs at the time and was a bit happy.

After they found the body they found 3 times the leathal amount of heroin in his body which means there was no way he would've been able to lift the gun to shoot himself. And he couldn't have shot himself and then injected the drugs because it was an instant death by the gun as well. There were no fingerprints on the gun while Kurt didn't have any gloves on.

Salaam Aleikum,
Sebastiaan