While Deadpool’s healing factor is already pretty disturbing, Venom actually has a healing factor that tops it in terms of how twisted it is. Weapon X transformed Wade Wilson into the superpowered Deadpool by engineering a healing ability for him that would replace the cells that his latent cancer would kill. This can lead to some messed up outcomes if anyone other than Deadpool tries to use his healing factor, but Wade is able to use it well enough despite its disturbing nature. Venom’s ability is a completely different story.
Venom is the name of the symbiote that most famously bonded with Eddie Brock to become an antihero. While bonded to a host, the Venom symbiote grants them a variety of abilities, including enhanced strength and stamina. It also constantly evolves new abilities as time passes and it bonds with new hosts. Despite the various hosts this symbiote has had, it views Eddie Brock as its favorite, which perhaps explains why it was able to develop its most powerful healing technique while bonded to him.
In the comic Venom: The End #1 by Adam Warren, Jeffry “Chamba” Cruz, Guru e-FX, and VC’s Clayton Cowles, the Venom symbiote tries to keep Eddie alive as centuries pass and his body is ravaged by old age. While Venom is able to replace most of Eddie’s failing organs with symbiotic replacements, his brain proves to be more troublesome to preserve. Venom is only able to keep his brain alive by replacing it one neuron at a time. This has the unsettling side effect of replacing everyone in Eddie’s memories with a “Venomized” version. While this does keep Eddie alive, it does so at a terrible cost.
Life is often said to be the sum total of all one’s past experiences. When viewed in that light the method Venom uses to keep Eddie alive is actually just killing him in a different way by slowly erasing these experiences. While this is disturbing enough, on top of this Eddie seems to be suffering the whole time. He is essentially an empty husk throughout this process, but there seems to be suffering in his eyes and his mouth is frozen in a silent scream. As the comic's narration notes, Eddie is being kept alive whether he wants it or not. And from the looks of things, it seems he most certainly does not.
While Venom keeps Eddie alive out of love, it is clear that this prolongation of his life actually causes his host a lot of unnecessary pain. While the actual method of life preservation is disturbing enough, the disparity between Venom’s intentions and Eddie’s suffering is arguably even more twisted. So when it comes to whose healing factor is the most disturbing in Marvel Comics, Venom easily beats Deadpool on every level.