Neville's reason is actually quite logical. As far an the general Monroe Republic knows, Neville is a disgraced officer. Plain out killing Monroe makes Neville into a traitor, and loyalists will go after him. Neville would set up a show trial, but it would "justify" the killing of Monroe to the Republic, and especially the military. ANd then Neville who "brought Monroe to justice" can "legitimately" take over. But of course Miles comes out of nowhere to free Monroe...who I agree, needs to die. 1st half of season 2 will be fine.