Shane was going to kill Rick. Shane left him no choice. Either he killed Shane, or Shane kiled him. Rick was absolutely heartbroken for having to do it.
Besides, I disagree. Maybe one day you'll meet the right one, a girl that isn't some cheap slut, a hoe, as you call them. (Altough with that attitude, I doubt it.)
But I'd rather give up a friend, than the love of my life and my children.
Lori? Manipulative? Not on purpose.
Shane comforted and watched after Lori while she was an emotional wreck.
The world got run over, she had to flee with her son, but her husband was in a coma.
There shows up her knight in shining armor, Rick's best friend, Shane. She was already close to him, as friends. Shane also knew Carl.
It's only naturally that certain feelings would grow. She wanted to provide herself and Carl with a stable family, as far as that was still possible in a world like this. They needed it, emotionally , as well as psychically .. for protection.
Then, her husband, the man she loves enough to marry shows up. He woke up from a coma, risked his life all alone to save strangers right away, and to find his wife and son.
Of course she was going to get back to him.
It was already back then that Shane tried to kill Rick, because he was jealous. Jealous because Rick got Lori and Carl. Jealous because he felt his leadership position was treathened.
And he went emotionally insane, because... he has forgotten about the world as it was. He has adjusted to the zombie world. And then, suddenly, Rick enters. In Shane's mind, that's not right. It doens't belong there. Rick was dead. Shane's mind, chaos.
Shane was going to kill Rick from the start. Rick knew it, eventually, but tried to convince Shane otherwise. Passively and agressively. In the end, he was left with no choice. He was at the end of a barrel, and saved himself, his family aswell as the group.