I think the first one is most likely as well, especially because of accounts of him having major health problems in not only his later years but also in the weeks leading up to his death.

It makes a better story to say that the great warlord died after a ninja dishonorably stabbed him while he was on the can than to say he had a seizure and died on the can.

Maybe not glorifying Kenshin, but which would you prefer to be able to tell people? The seizure might get a chuckle, but the assassination would get a much greater response.