If all Gods are Chaos Gods ultimately (see horders of chaos book) doesn't that mean that indirectly all the people are the Warhammer World are indirectly worshipping at least 1 of the great 4?

If this is so then why do the Chaos armies slaughter all through Kislev and the Empire? Surely all the inhabitants are worshipping Chaos Gods anyway, so basically the Gods would be killing their own followeres.

Any thoughts?