In Macbeth how could Ross be seen as a character that plays both sides?
Rather than being particularly deceptive, it seems that Ross is one of several noblemen in the play that is simply attempting to adjust to the realities of the events that surround him. As he says to Lady Macduff, in times like the ones described in the play, the nobles of Scotland and their families "float upon a wild and violent sea."
Ross indeed is at the center of much of the politics in the play. On the one hand, he delivers news of Macbeth's promotion to Thane of Cawdor, and (unlike Macduff) plans to attend Macbeth's coronation at Scone. On the other, he is the person who reports to Macduff that his family has been murdered:
Your castle is surprised; your wife and babes(235)
Savagely slaughter'd. To relate the manner
Were, on the quarry of these murder'd deer,
To add the death of you.
Ross had encouraged Lady Macduff, his cousin by marriage, to flee, and he struggles to give Macduff the news, saying first that his family is well. This seems due to a natural unwillingness to break the terrible news to his kinsman rather than dishonesty or scheming. He is also obviously on the side of the noblemen when they rebel against Macbeth in Act V, and he also informs Siward of his son's death, the second time he has had the duty of bringing bad news in the play.