Root canals are absolutely evil (in my opinion), because it means keeping dead material in one's body. Dead cells are always in a state of decomposition.
More precisely, a tooth is filled with numerous small canals and pockets (since obviously it was made from living tissue, and that is never a solid block). Those pockets are sealed with a root canal treatment, and often bacteria are trapped within these pockets, since it is virtually impossible to completely clean a tooth before sealing it up. If these are bacteria that can live anaerobically (meaning, without air), they do live on and produce their waste inside of the tooth - which are toxic to the human body (as most bacterial waste products are).
Nowadays there is speculation that many of the immune diseases appearing later in life are a caused by these toxins building up over the years and constantly stressing the body.
If I recall correctly, it is not even necessary to have a clinically obvious root canal infection. Even a normal looking root canal tooth can harbor small amounts of bacteria that are constantly producing small amounts of waste products.
It's been a while since I did my research on this topic, so the details escape me. All I know - never ever get a root canal, if you don't want to slowly poison yourself.
This article confirms that it is anaerobic bacteria that can be found within infected root canals: https://ncbi.nlm.nih.gov/pmc/articles/PMC3324152/
And in this article there is more info about this topic: https://www.biodentistry.org/wp-content/uploads/2020/06/Root-Canal-News-Release_2.pdf
Weston Price is a physician who did a lot of research on teeth, but his findings have been systematically suppressed. It's worth reading his book "Nutrition and physical degeneration", which is out of print, but probably available in digital copies on "the internet".