Why did he say "Mos Yumas"?
Ohr ha'Chayim: He will die bi'Yedei Shamayim; or he will be killed bi'Yedei Adam.
Why did he say "ha'Noge'a"?
Ohr ha'Chayim: One is liable for a Yisrael's wife even in a way (Lo k'Darkah) that one is exempt for a Ben Noach's wife.
Ha'amek Davar: It cannot mean to harm him - if so, why did he command so only now?! Rather, Avimelech revealed his hatred - he forbade anyone to do business with Yitzchak, lest he comes to put his eyes on his wife. This should have harmed Yitzchak (but HaSh-m blessed him, like the next verse says).