Akhu and Muuet:
The Akhu and the Muuet (also sometimes called mut but not to be confused with the netjeret) are both forms of human souls/spirits which exist in some form after death. Akhu and Muuet have big differences between them, or rather between how their afterlife is going.
Akhu are those souls/spirits which have been judged against the feather of Ma'at before the netjeru, and found to be equal to her feather (I will explain the concepts of Ma'at at a later date). This judgment occurs at or during the 70 days of mourning which take place in the physical world after the death of a person. During this time, the dead man is not considered an ancestor yet, and doesn't receive the same treatment of the Akhu. Rather, he/she is mourned and prepared in every possible way for the journey through the Duat (underworld), and heka is worked for them.
After the 70 days, one can begin placing offerings to the dead man as an Akhu, and begin to count them among your ancestors. The reason for this is so that the dead man is more easily able to become an Akhu, and so that a person doesn't accidentally attract a Muuet to themselves.
Without waiting the 70 days, it is possible for the dead man to become confused, unable, or unwilling to become an Akhu, and because of this, they can become a Muuet. It is because of the attention paid to a person's spirit/soul upon death that they can choose to hang around, and for some, this will be what keeps them from moving on and becoming the spirit that they are intended to be once passing judgment. This isn't to say that someone cannot hang around after death and still be an Akhu, rather, they have to first reach judgment and then can easily return to the physical plane as a spirit to communicate with the people left behind and/or to watch over them.
By distracting the person's spirit from moving on and going to be judged, you prevent them from reaching the afterlife as they are intended to. If a dead man is kept from the judgment long enough, whether by choice or by interference, his soul/spirit can become corrupted and he will become a Muuet.
An Akhu will never become a Muuet, because they have already reached judgment and passed all the trials that are required for one to be an Akhu. However, in the span after the person dies and before they reach judgment, they can become a Muuet.
A person can become a Muuet through several ways, by choosing to avoid judgment, by being confused away from judgment (as was stated above), by being refused judgment, by being without a guide (either a spirit or a tome) through the underworld, by being forced or convinced not to go to judgment, by not being given the proper offerings, by not being cared for properly upon death, etc. There are several things that can go very wrong from the moment you die until the moment that your ab is weighed equal to Ma'at's feather. Any one of these things that can occur can lead to you becoming a Muuet, or worse yet, to your complete destruction. There are places you can become trapped and die a death a million times, other places where you can become trapped in a rotting corpse, others where you are trapped in your own khat (physical body). It was considered that if everything did not go just right, that you would die permanently, or that you would suffer a fate worse than death (one of which is becoming a Muuet).
Muuet are beings which are highly dangerous in that they are very violent, and can attach themselves to a person, wrecking all kinds of havoc on that person's life. The Muuet are dangerous in that they are normal human spirits who have been corrupted by not being able to reach judgment, and because of this they lose much of their humanity and become violent and uncaring towards others. What becomes important is only their goal, which can be anything from protecting a place to receiving offerings to killing a person or people. They often simply cause bad luck to come to a person, however oftentimes they can cause death if they are attached to a person too closely for too long, or if that person will not heed their warnings.
A Muuet can choose to attach themselves to a person or a place, however if that Muuet was forcibly created, they will often be forcibly attached to a person. Yes, a Muuet can be forcibly created by a hekau, then attached to a place or even an object to guard it, however it is more common that one becomes a Muuet by choice and chooses to do this. To my knowledge, while their are instances of netjeru setting conditions up for one to become a Muuet, they do not have ultimate control over the creation of a Muuet, as the dead man can do certain things to go against the wishes of the netjeru. If someone becomes a Muuet out of a force outside of themselves, they will tend to be more dangerous than the average Muuet.
Muuet can be removed from a person, but this takes a hekau with the proper knowledge (of Muuet and their removal), otherwise the Muuet can become more violent and hold more strongly to the person. In most cases, appeasing the Muuet and attempting to prepare and bring them to judgment (so that they can become an Akhu finally) will be enough for a Muuet to release their hold upon a person.
A Muuet can become an Akhu if they manage to find their way to judgment, however they must be in the proper state to receive judgment, otherwise they will remain a Muuet or destroy themselves in the attempt. This is why the assistance and removal of a Muuet should be best left to those who understand and work "with" the Muuet.
They are wicked, doomed souls, but not without redemption, and shouldn't be approached by the unknowledgeable and unknowing. Their wickness is not entirely their faults, as even when trapped in the misdeeds of their former lives, their spiritual existence is of a different and separate nature than their physical existence, and they simply have not realized such. If one attempts to remove a Muuet, they can easily attach that Muuet to themselves accidentally.