Carbon monoxide both displaces oxygen and actively scavenges it from the bloodstream, inducing hypoxia quickly and painlessly.

Carbon dioxide is responsible for the burning feeling in your lungs when you hold your breath, and acidifies the blood resulting in the "shortness of breath" feeling when exercising.

If you breathe pure CO, you would never feel short of breath, you'd simply get a little light-headed, get tunnel vision then black out.

If you breathe pure CO2, your lungs immediately scream in agony as you involuntarily gasp and struggle to catch your breath.

You can get the tiniest little taste of this experience by taking a deep whiff of a freshly opened container of soda.

tl;dr Carbon monoxide is humane, carbon dioxide is literal torture.