It is their doctrine that conflicts with Biblical scriptures. They teach that God, The Father, was once a human. This is in total opposition to everything in the Bible.
Additionally, they have a practice of holding religious marriage ceremonies for their dead ancestors, who have no record of a "church wedding", but only a civil record.
Again this is directly opposed to the Biblical principle of individuals being solely responsible for their own salvation. They believe that their ancestors who married civilly, but not under God, are in some state of limbo, and that by performing a ceremony on their behalf will aid in their salvation and free their soul.
There are many others but really, I agree with your initial statement that by some definitions all religions are cults. From an outsider's perspective, It is easy to argue that The Bible may well be the most evil and harmful thing in all of history.
There are several tiers of debate, at work here.
Is there such a thing as God at all?
If we can agree that God exists, is He necessarily the God of the Hebrew Bible?
If we can agree that he is, then, and only then, can we begin to debate theology, doctrines and interpretation issues.
Etc.Etc.