The thing about bogs is that they're a great place to put something you don't want found. Like a body. Bogs are in a few places across northern Europe, and the vast majority of bog bodies are of people who died violently.

So it could be bodies of people killed by mob violence, like a thief that came to town. But also people robbed. The point is to get rid of a body and not have a reason to bury them like everyone else in town. Even if we entertain Occam's razor a bit, ritual sacrifice that was similar across unconnected places vs. run of the mill human violence makes the sacrifice angle hard to accept as the singular answer for 1800 bodies.