No, you over-simplify the matter. Among the criticisms of Union generals McClellan and Buell was that they were soft on slavery and perhaps had intentions of restoring the Union to the status quo ante bellum. Some soldiers expressed dismay when Lincoln issued the Emancipation Proclamation, arguing that they were fighting to preserve the Union rather than fighting to free the slaves (and they probably used a pejorative not acceptable in polite society).
Certainly as the war progressed, the overthrow of slavery gained importance.
McClellan was a coward and traitor pure and simple. Furthermore, he ran against Lincoln as...a Democrat!