Will you be hable to kill them?

i supose yes , youll be able to kill most things hopefully

I hope so.

Im so sad that Harold basically died in F3. He was one of my favourite characters. But knowing gamesas, they will probably do something incredible and change the circumstance to their advantage. Now that their leader and god is dead, maybe there will be people dressing as ghouls and glueing plants on their heads. Or even ghoulazing themselves purposelly, trying to imitate Harold?
Death is certain, Life is not.
-Augustus Hill

i hope there is no invulnerable npcs , and i believe fool is set between 2 n 3 so harold should be still alive

This

Yes, Fallout Online is set between 2 and 3.

I think.

If it was between those two, we would have NCR vs BoS. You are being served by Nymus powered by black boyshorts with black lace, a pink bow, and pink ribbon and a hot pink and black lace, totally frilly.

Oh? I wasn't aware of that. I quess that's a good place to put it. Now we can Harold again. WOOO! XD
Death is certain, Life is not.
-Augustus Hill

There's a quest option to not kill Harold in FO3, so it's possible that the devs could treat him as not dead but definitely rooted in place.

ooooor, There are not Followers of Harold and the entire FO3 doesn't exist in Fallout online. Maybe I missed something, but I've heard a number of times that gamesas will have nothing to do with FO3 because it's made by the Other Company. I suspect while gamesas has a license to make FO online, they have no license to use any property created by the Other Company. This means no Followers of Harold and Harold can't become tree man in the gamesas time line.

Or there most likely will be followers since we got a newsletter containing nothing but info about the church of harold :/

And even so, if you read fallout bible (all fallout info is in there) you can see that Bob "the tree" is consuming him more and more by the years and it even starts to grow fruits etc etc.