.Edit: I just restarted the game, seems to have fixed it. Guards agro now. Saving and Loading didn't work.

Saving and Loading didn't work. Restarting the game worked and fixed it for me.

I'm on PC, had this same issue. Except I was in Sciropeshire and ran into the Zealot there. Killed his horse and he fell off, then he stood there and let me kill him. His health also kept regenerating if I stopped attacking him, Eventually set him on fire and kept my damage on him and he died. Now nothing is agroing for me. Even guards in hostile areas, they just stand there and look at me and go on their merry way.

Yeah, this isn't a bug. Off to the right hand side I believe, is a stairway leading upstairs where Sigurd, Basim and Falke are. Go up there and it will trigger the cutscene.

This just happened recently. I just got into Sciropescire and have been doing a bit of exploration and just realized when I ran into the Zealot in that area, that nothing is attacking me. I knocked the Zealot off his horse and he just stood there and let me defeat him. I thought it was a bug, but now when entering other places where guards are, they are not attacking or agroing me. Even if I attack them. They just stare at me and stand there or continue on marching off somewhere.

I'm on PC.

Yep, same here. When you interact with him outside camp, it brings you to the Jomsviking recruit screen and the recruit button is greyed out as well. It's treating him as a recruitable Jomsviking.