I strongly think that Shane wasn't going to kill Rick, but instead have Rick kill him.
So to start the scene off, we see where Rick admits to Shane that he knows this is a murder plot. Shane has a normal reaction to it, they had a conversation before Shane drew his gun. If he was as clear-cut as he came off as, he would've surely shot Rick down right then and there.
Remember, Shane executed his plan without hesitation and fail. He led Randall out in the woods and brutally killed him without a second thought, perfectly convinced the group of his armed escape, and had the men searching the woods. That dedication surely would've been shown in his goal if he really was going to do it (Kill Rick), but he didn't.
Also during Rick and Shane's altercation, Shane consistently rambled agitating narrative to Rick, about him not being a good leader, a good father, nor a good husband. Even going as far as to call his wife "Broken" and his son "Weak", in hopes of infuriating Rick. He also insisted, more than once, that Rick raise his gun. This isn't something a murder plot would consist of.
Finally: When Shane was fatally stabbed in the heart by Rick, he was at ease with it. He didn't fight back at all. Shane says "It's ok" to Rick and brushed Rick's face as a farewell while he bled out, knowing it'd be the last time he'd see him. Remember, this is Shane's best friend since childhood, high school, college, and the police academy. Rick turned to him when he had problems in his marriage, Shane was an uncle to his kid, Rick and Shane were practically brothers.
Shane knew he couldn't be there as his psychological state was taking turns for the worse and his mind was spiraling out of control due to the constant stress from the group, from Lori, Carl, how the rest looked at him, and his belief that Rick wasn't making good decisions for everyone. John Bernthal, the actor for Shane, also confirmed that Shane was aware that he was losing himself.
(Also, a side note, my theory was influenced by Bernthal's preferred way for Shane to go out, where in the end after Rick killed him, it would turn out that Shane didn't even have the gun loaded. That says something.)
