The Rambam poskens like Rava that an omer mutar is karov l'mezid.
In hilchos Shabbos a melacha done not knowing the act was assur is considered B'shogeg. Why the apparent contradiction?
Daniel, Windsor, Canada
In both places where the Rambam discusses Omer Mutar (Melachim 10:1, Rotze'ach 5:4) he is discussing the law regarding killing and adultery of Nochrim by Nochrim. It therefore seems that the law of Omer Mutar is not something that the Rambam is ruling on across the board, but rather in very specific cases. (See for example the Gufei Halachos on the Rambam in Melachim ibid.).
[In addition, Tosfos (Shabbos 68b, DH "Aval") says that the laws of Omer Mutar are unique to killing.]
All the best,