If the dental disease spreads and causes further infections in the dogs body and blood then yes the dental disease if left untreated might kill your dog.
Some dogs live a full life with dental disease but then some dogs suffer further infections and die as a result of the dental disease that spread.
If possible it's best to get the dogs dental disease treated to prevent further infection but I understand some people even though they love their dog dearly are unable to afford to take the dog to the vet for dental work.
That goes for other medical treatment and checkups of the dog as some people cannot afford vet visits even though they love their dog and wish they could.
But if you can I would recommend having the dogs dental disease treated by the vet to prevent further spreading of the infection.