First off, this is not a hobby this is an addiction now. I have read the first five books several times now. Too much time? Nah, time well spent.
Although i want to hate snape, i cant.
because i cant help but think maybe snape DIDNT kill dumbledore.
What about the unbreakable promise that happens in the start of the book. If Draco fails Snape agrees to carry out the task. Draco did indeed fail as he droped his wand. He couldn't do it. He just wanted to prove to mom and dad he could and save thier lives. But in the end he could not betray them.
Draco also was about to join Dumbledore. Even after the death eaters showed up Draco was noticeably upset Greyback was in the school. He dare I say felt guilty. Dumbledore was making progress with him when they showed up. Then Snape who could have taken them, they had thier back to him.
The person who said Snape was arguing with Dumbledore was Hagrid. He over heard them when he was coming back from the dark forest. He was visiting his monster spider.
This argument could go 2 ways. One you could jump to conclusion as you say, this was all planed. The other way is Snape was tired of no one trusting him because he was a double agent on both sides. He was wanting to just stop. Dumbledore saying, you agreed to this now do it. Snape says fuck this an chooses a side, the dark one.
The unbreakable promise is not to be writen off. If he did not fullfill the promise he dies.