Clarifying a few facts:
1. We don't actually know what the motive or circumstances were. It could be totally unrelated to the events of that afternoon, to drugs or to money. For example, perhaps he tried to rape her and she fought back?
2. The intent was never to sell her $300 worth of pot. The story according to Source 1 is that they would be paid $300 for dabs made from marijuana the perp would give her. This is not an unusual transaction, it only has unusual players.
3. Dabs do not take any special skills or chemicals to make, it is just tedious to get right and time consuming to prepare. You can do it with a fireplace lighter and a sheet of tin foil, and the very helpful YouTube even has tutorials to show you how.
4. There is nothing to indicate Kaytlynn was into drugs. She had reportedly been smoking marijuana with some friends. I personally do not categorize that alongside dropping mollies or doing heroin, or even drinking jello shots, so imo she was a normal kid. I'd say sneaking out to get drunk when I was 14 was far worse than K smoking a joint on her own couch.
5. There is nothing to indicate the deal was made, that K got the weed, or the money.
6. The entire story of the deal and its details is hearsay from Source 1, with the only hard evidence being phone pings and a text message between perp and K.
7. K was brutally murdered to death with a hammer. This is not an OD or tox report suspense as far as we know right now.
Hopefully this helps put the focus back on the questions these leaves us, which is why did he do it and how much time will he get for it?