The existence of the Widderslainte is a living argument against free will. As unawakened they are sociopaths and monsters: an unusually vicious bully, the child who tortures pets or the source of all those missing young girls, if and when they awaken the fun really begins.
Even if they don't contact their colleagues they're a Nephandi in all but name and tend to spend their time performing atrocities on their own. If they do make contact then you have another bastard to the ranks of hell who was destined to piss on the world from the start. Although on the bright side barrabi tend to resent the widderslainte so there is always the chance that some backstabbing will occur.
Occasionally the other factions will find the budding psycho first and attempt to save them, after all they haven't gone through the caul yet. The problem is they don’t need to go through the caul, in a sense they’ve already gone through the caul and many believe that any attempts to redeem them are doomed to failure (and that they’ll happily drag others down with them). The Avatar of a Widderslainte has been irrevocably twisted in a previous life; save perhaps for the highest orders of magic from the Exemplars, Oracles, and/or Archmages.
The current human attached to that Avatar isn't inherently irredeemable, however. You could theoretically keep one in check. But you'd be constantly fighting their Avatar for control, and any magic the Widderslainte uses will be twisted by their Avatar. If you could separate the Avatar from the Widderslainte, there's no reason the child couldn't be rehabilitated (though that isn't exactly an easy feat).
Most of the time, since the ability to separate an Avatar without killing the human (or worse) is rather rare, and the ability to un-invert a twisted Avatar is practically myth, there isn't much most mages can do, and there's still the psychological mess you're left with afterwards.