They're definitely dangerous.  They'll kill and even eat humans and seraphim if they can catch them.

[Hngh, but how to explain the complications involving Malevolence...  Eating the other cookie he pulled from the bag a few minutes before, Dezel tips his hat back again, green eyes looking out over the scenery in front of them as he folds his hands over his middle.]

More often than not, no.  Especially with plants, animals and seraphim.  Some humans who turn are unknowingly consumed by their impurities, but once that happens, they accept what they've become and revel in their corruption.  They love the power too much or believe the atrocities they commit are somehow for the best.  As a result, they become impossible to purify and have to be slain.