...the text of that website mentions the resurrection and the return of Kira. Which would mean that Kira existed before and also that his followers are a group which has existed already and believes that Kira now has come back. While this might be true, first you have to remember that:
- Not much is known regarding the legends, myths, beliefs and religions that existed in the world where Death Note is set. All we know is that it is similar to our world. It would therefore be too hasty to immediately conclude that a cult believing in Kira before Light used the Death Note is already in existence. It might or might not be the case.
- In addition, there is no way to verify the existence of Kira's followers or Kira before Light had the Death Note as this was never explained nor explored in the manga. All we know is that websites talking about Kira as the 'messenger from hell' were suddenly put up after the news picked up on the mysterious killings.
- Furthermore, it would not be difficult for people to make up stories given the imagination we have. Anyone who knows how to make a website with nothing better do can make a website about Kira. Why then do multiple sites talk about Kira if people can make up their own stories? It could be the Kira myth or legend started by one website was found to be popular to most so other websites copied them and the popularity spread even more.
So you see, we can't assume yet that these 'followers' existed way before as they could just be a random group of people who have their own motives or objectives. There's many possible reasons why they would do that.
It is possible that Kira, that is, a person who held a Death Note before, could've existed. This is because there are rules in place regarding the ownership and usage of a Death Note whenever it falls in human hands. If this is the case, then it is also possible that a group of people who knows nothing about the Death Note worshiped this person because of his/her power. It is also possible that this cult survived throughout time and interpreted the mysterious killings as a sign of Kira's return. In order to spread the 'Word' about their religion, they might have put up a website, which could reach more people.
But again, this is all just possibilities so we cannot definitely say nor conclude on anything unless the mangaka would reveal them in the future.
To conclude, did Kira (a person who held a Death Note) exist even before Raito Yagami started his actions of justice? Maybe yes. Maybe no. But the possibility that he existed is strong due to the presence of the rules regarding humans obtaining Death Note ownership. Did followers of Kira also exist before? Maybe yes. Maybe no. It would be too early to say without evidence that they existed as Kira's 'legend' could just be a made-up story in order to gain more website visitors or for any other reasons known only to the person or to that group of people who created/started the 'legend'.