Plus, sometimes people just get angry and act in strange, irrational ways. Or lash out for no reason. Especially when it's the end of the freakin world. I hardly expect everyone to be charming and likeable in that situation, and find it strange that somehow fans expect that she SHOULD be.
However, in one episode she basically begs Rick to handle Shane and then gets pissed last night when he did.
Perhaps she didn't expect him to murder him but she essentially set the whole situation in motion by:
1. Telling Rick that Shane was becoming a huge problem and telling Rick to handle it
2. Giving Shane a false sense of hope about their relationship with that apology in the prior week's episode.
Was she being manipulative of both of them or just plain stupid?
In any case, for her to get angry at Rick for handling the problem with Shane and that he did try to kill him
I think it's bizarre for her to react in the way she did.
Also for the writers to explain her behavior as, "Lori being angry at herself,' was just idiotic.