Why is it that the only people who believe that the Union cause was over slavery are the Confederate supporters?
Just stop. Your pretense has worn thin. If you bring up the topic of slavery, you are effectively arguing that it is justification for the Union's invasion of the South.
If you don't want to argue this, don't bring up the topic. It has no relevance to the Union's reasons for invading. It is just an ex post facto rationalization for what they did.