When we sin willfully we give the enemy and those who aren't on God's side a reason to mock God. When we sin willfully we are opening up our own lives to the enemy. When we sin willfully we are letting the flesh rule rather than the spirit.
"You who boast in the Law, through you breaking the Law, do you dishonor God? For 'THE NAME OF THE GOD IS BLASPHEMED AMONG THE GENTILES BECAUSE OF YOU' Romans 2:23-24
Sin is sin. It doesn't matter what that sin is. We humans are so good at pointing out sins we think are so bad but usually if we are in the habit of pointing out sins in others those sins are ones we have no problem with (or we really have a problem with them in our hidden lives).
But if we were to pick out a sin that is pretty bad, we could look at the one Jesus mentioned several times -- religious pride. When we become complacent in our faith, in our lives, we can easily fall into religious pride.
And that is one of the ugliest sins in existence.