Aside from what BetterNameUnfound posted (and that's the most honest answer), the nitty gritty answer is that Sue probably has the most raw power ability to kill the rest of them if she did something nasty like put force fields in their brains (she might have to suffocate Reed instead, though)... but if we're talking about "things someone could do if they acted in a way they never acted," Reed is fully capable of building a gadget to kill any of them, Johnny could easily roast Sue or Reed if he struck first (he might have more trouble with Ben), and by the same token there's not much stopping Ben from just randomly smashing Sue or Johnny's head if he wanted to.

(It's still not as bad as the doing the same conversation with the Justice League, though. I'm pretty sure every core member of that team has trashed the rest of the team at one point or another).