I thought that was strange, too, but I can't think of a reason why he would say that. Maybe it has something to do with the whole time-travel thing??? Who knows.
Could he be alluding to all the knowledge he has of the island and how he's told Locke he could "show" him things he could never imagine? If he's dead maybe all the island secrets will die with him?
or he could be saying were because the biggest threat to the island was Dharma who WERE on the island till Ben purged them all. I think they are part of Dharma and that's why Ben uses past tense. It makes perfect sense. Anything else makes no sense because why would these people be considered a past threat but not a current threat? Simple answer is Dharma is no longer a threat because aisde from these four no one else is on the island. Were means all the people who were killing the natives, who Ben purged.
Or, it could be because their main purpose for coming to the island was for Ben. Once Ben is eliminated, the freighties no longer have any real purpose for being on the island, hence they 'were' a threat, but they would no longer be a threat at that point.
and
and 