Holland grew up alone, abandoned in a shack, trying to learn how to control his unnatural form. Banshees are violent creatures that feed on the souls of the weak if left without leaning how to control their urges they'd kill their victims ruthlessly.
Murder is like a drug to banshees, once done it's urge is hard to rid off Sam found Holland feeding on a dying witch and directly cast a caging spell to stop him. Sam reluctantly started casting a death spell before stopping as to noticing the young MALE banshee
Sam took him in after healing the witch, and helped him learn how to control his urges. The only way for a witch to train other creatures was for them to be unbelievably powerful, which is something Sam never lacked.
Sam comes from the bloodline of powerful royal witches that have even faced death and won. They possess all sorts of magic and have created new magic themselves. Holland to this day has never murdered anyone although the urges skyrocket when his hunger is induced.
Witches have created the all powerful supernatural creatures, they are the only creatures that were naturally born to lead the world into peace and harmony. But of course as any population of good people come evil ones, many created new creatures in order to build an army and take over the world
Many failing and some succeeding to create fear in peoples minds once their names would be mentioned. The SuperNaturals would never allow evil to spread and would work on keeping the peace just like the ancestral witches would have desired