Shane did try to save him, and from the way they showed it, it was obvious he wasn't going to get Rick out of the hospital. He put him back in his bed, and did his best to secure the door. I thought that was good enough given the situation. So after leaving, he probably did assume he was dead. Don't blame him. He did develop a relationship with his wife, but under the circumstances, don't blame him for that either.
When Rick got back, he stepped out of the way completely, while Rick pretty much took over and disrespected Shane's opinion at every turn. Then Lori treats Shane poorly which only adds to his stress. It's obvious he loves her at this point, deeply. So zombies are walking around on top of all this which enough to drive anyone crazy. Shane couldn't handle all this so he starts going crazy. Can't say that can be blamed on him either by the time it kicks in. It took a long time to build up. Go back and watch the first episodes. He was very much submissive to Rick and respectful of Rick and Lori's relationship until the stress of everything started getting to him.
So this brings me back to my original argument, which is that MAYBE all this wouldn't have had to happen to Shane if Lori and Rick didn't treat him as they did. Lori should have told Rick immediately about what had happened in his absence. She should have confronted Shane over why he told her Rick was dead and let him explain himself. Rick should have not been such a pansy in waiting so long to confront Shane about would had happened with him and his wife, especially if he clearly wasn't going to hold it against him. Why be a douche and not say anything and let your supposed best friend torment himself over everything? The same goes for his wife, since Rick said he supposedly had it figured out before Lori told him. Why would he let his wife and best friend anguish in guilt when he could just say, "Hey, I understand how this could have happened. I love both of you anyway and don't harbor any ill will."
At least Shane tried to do the right thing, which was stay out of the way, let Rick and Lori become a couple again, and allow Lori to be the one to break the news. He also put down his own desire and opinions repeatedly out of respect for his friend, Rick. When did Rick ever do any of this for Shane or his own wife amd son?
Shane is the one who got screwed in all this, in a rather Shakespearean and tragic kind of way. I don't feel sorry for Rick at all. He even refused to take ownership for any of it when he killed Shane by telling him it was all on him. What an arse.