First, let’s just establish the fact that Karkat could be wrong. While he is firmly stating that he believes Jack Noir to be the cancer he created into this universe, he gives no support or firm reasoning. This leads me to believe that Karkat is defining what form he thinks his cancer has taken based on the assumption that Jack Noir is the cancer. Jack Noir, as the primary destructive force in the kids’ Sburb session, would seem to be the most obvious person to take the cancer’s direct form.
However, the cancer could still be something else - perhaps, something that caused Jack Noir’s actions and destructiveness in the first place. The cancer’s form might not be “ALL CONCENTRATED THROUGH THE ACTIONS OF ONE HOSTILE AGENT IN THE SYSTEM, WITH AN INSTINCT TO DESTROY EVERYTHING IT HATED,” it could just as easily be something else, and Karkat could simply be guessing - with the “certainty” that a leader needs - as to its true identity and nature.
What if instead, the cancer of the universe was a bit more like a real cancer? Cancerous cells in a body multiply not out of conscience, not out of active bloodthirst or destruction. They are simply cells doing what they were made to do, only, they were made wrong. A mutation convinces the cells that they need to multiply, without inhibition - in order to help the human body, even. They aren’t aware that multiplying actually does damage. The intent behind the multiplication and the sequence of DNA that makes it happen is to heal the body, but uninhibited by the unmutated sequence, it hurts, and then eventually, kills.
I think that the cancer could actually be…
To expand more on your point of jack noir I would like to introduce a chain of “cancers”
Jake English (JE) Made the bunny with the weapons -> Jade Harley (JH) had Bec send the package to john -> Dad Egbert (DE) had in in the car -> Rose Lalonde (RL) dropped the car due to a bad internet connection -> Authority Regulator (AR) deemed the car illegal and therefore confiscated the package -> Pedigree Menicant (PM) came to retreive the package, which jack may have been oblivious to the contents (as expressed by page /?s=6&p=003456) -> Jack Noir (JN) then used the package to start his rampage. TL;DR, Jake, Jade, Dad, Rose, AR, and PM could all be charged with either being cancer, since cancer has more the one cell.