In world religion, there are generally-speaking two perspectives on the world:

Firstly that the world is a creation of benevolent god (or gods) and its many contradictions actually create harmony through polarity. Good vs Evil. Dark vs Light and so on. Examples of this kind of religion are Christianity, Judiasm, and Islam.

Secondly, the world is unharmonious and created by an evil God and we are under the illusion that it is harmonious. This is the perspective of Manicheans, Gnostics, Cathars and Weavers.

The Weavers were Gnostic in belief and considered all of creation to be a (Manichaen) struggle between Good and Evil. The world we inhabit being the domain and creation of Satan. Outside of it is the unseen world of Spirit, the realm of the True God. Satan has tricked ‘pure spirits’ (katharoi) into the world and woven them into prisons of flesh, ego and consciousness otherwise known as human beings.

As the world is a creation of Satan, to a Weaver, all that was material was seen as evil. For a Weaver, evil quite literally was everywhere and in everything. However, there was an way out of this hellish prison-world of evil. If adepts could avoid yielding to evil, the temptations of their lustful dirty body (carnality, selfishness, pride etc) they could be released from earthly bondage and return and reunite with God after death.

Obviously the simpliest way out would be suicide. But unfortunately, like Buddhists, the Weavers believed in the transmigraton of soul or reincarnation. Killing oneself does not free one from suffering but actually just plunges one back into the world of suffering - often in a worse condition. This is why they were strict vegetarians, because their souls could end up reincarnating as animals. (In fact, at the time, an easy test to discover if someone was a Cathar was simply to offer them red meat. If they refused, they were condemned and usually tortured)

The alternative is freedom from base matter through exhaustion of the flesh: either through asceticism (ascending path) or debauchery and depravity (descending path). Only this goal of freedom was considered worthwhile by the Weavers. So morality as we understand it, was baseless and irrelevant. From the Weavers point of view, the Devil has already won and he rules over a world teeming with his slaves. So anything became permissible - lies, treachery and even murder on a grand scale if necessary.