Turned start out as vamps. They're infected, get the whole 'immortality' thing, etc. A turned automatically gets the Eerie Presence flaw (See Vices, Flaws, and Faults).
Nature
Turned are the darker form of vampires. There is no 'redemption', no bright side, no desire to do anything beneficial for anyone else unless it furthers their own gains in some serious/huge manner. Vampires of course, feed on human blood. But they don't kill their prey (at least not deliberately through feeding). A vampire that does kill while feeding, drains their victim dry, goes through a process of turning. He becomes more aggressive (a hell of a lot meaner), develops memory issues, manipulation scores increase, etc.
Cause / Addiction
When the victim dies a cocktail of hormones/chemicals is released into the blood which the vampire consumes. One of these is a high level of adrenaline which is addictive enough that most if not all Turned and turning deliberately draw the process out into various forms of torture to fulfill their craving. The transformation can take months or years depending on the host and frequency / amount of hormone ingested. The more the host is affected, the more he needs/craves, just like any junkie. The physical and mental addiction can kill the vamp/Turned in withdrawal, though very few maintain relationships close enough to attempt to 'rehabilitate' them.
Downward Spiral
Feeding on adrenaline laced blood provides a high to the vamp, in addition to the blood being assimilated faster toward healing. One of those 'I'm invincible and nothing can touch me' highs. The thing is, Turned don't give a damn about anyone other than themselves. While they're turning, they might be able to see what's happening to them, and they might try to steer away from the path they're on, but each slip brings them closer. Turned don't care about hiding their existence from mortals, they don't care about whose territory they might be going through or hunting in, they leave it for the vamps that do give a damn to clean it up. When killing mortals no longer gives them the high they need they'll turn to preying on their own kind, or on the were-community. In places where a ruling body has been established, Turned are almost always hunted down and destroyed for the good of everyone else.