The zombies come from necromancy practiced by underground scientists who use magic or science or both. Their goals are to successfully reanimate a person from a "fresh" death as well as to repair cells from grievous damage and strengthen cells in order for humans to endure and heal better. They are also interested in prolonging lifespans, both generally and of the terminally sick. To do this, they require various subjects and parts--some living, some already dead. All of the dead characters came from places like these. Everyone is basically an experiment gone wrong--then thrown away (or deanimated if possible). 

The scientists and doctors who work in these places may be motivated by personal interests (wanting to revive a comatose loved one, wanting to prolong their own lifespans), desires to advance medicine, or simple curiosity. Others may have greedier motivations or enjoy being able to inflict pain without consequence.   

Reapers do not approve of these scientists' antics and do what they can to squash them out. According to them, using magic to manipulate life & death could result in reality being offset and distorted with a domino effect. It could break the laws of the universe or something...  so they can't let it happen.

(To clarify, the reapers don't care if humans do manage to achieve better bodies and lifespans, and they also don't care about people being experimented on or tortured itself, but the use of magic to reverse death is where they take issue (for the mentioned reasons).

All current dead characters cannot recall who they were when they were alive (and lack all recollection from when). That's a common effect, which isn't ideal at all if you intend to bring back the person that was inside the body too. Sometimes these dead get to stay (remaining as experiments), but most of them are thrown away. Usually into the wight world. Others get incinerated. It depends on if they think they'll be noticed or not.

The only way to "kill" a dead one is to deanimate them. Attacking a dead person in a vital  When an Exterminator encounters a dead one, they don't do anything because there isn't anything they can do. All they really can do is report it.  (I'm toying around with the idea, but perhaps fire can destroy them. There is no longer a body for the "soul" to be binded to, and scientifically speaking, there is simply nothing there. There might be other components to this, though, if I canonize it.)

All dead experience an array of symptoms. The most common are chronic amnesia, (congenital?) analgesia, and dyschronometria; others including blunted emotion or apathy, anhedonia, emotional detachment, delayed reactivity, intellectual impairment, and acute amnesia. It's rare for a "patient" to exhibit only a few of these and extremely rare to experience none. 

Due to being targeted by reaper, there is a side goal of Medimortimers--that is, developing technology that disrupts the reapers' ability to detect other beings and to cause pain through doing so. Technically, they already have...