This looks obvious - but one of our new members at our modding site came up with the following and that got me thinking:

I'm thinking that Bal - assuming that is Molag Bal (prince of naughty plottings?) - had a purpose in teaching this skill or providing the soul gems that did not necessarily have the best interest of mortals in mind?

Well, I don't know where the idea that Molag Bal first taught mortals to create and use Soul Gems comes from, but it certainly fits his sphere. He is the Lord of Domination and I can think of no better embodiment of the master-slave relationship than someone capturing the very soul of another living being and then using their spiritual essence to further your own power. It's much crueler and more tyrannical than making someone a slave for their finite, mortal life and allowing them to be free in death.

Hircine seems to take pleasure in watching his beast-folk hunt, presumably watching mortal beings spiritually oppress and enslave each other gives the King of Non-Consensual Hug'n'Kiss quite the kick.

Souls are power.

Soul gems are intended to serve not only as vessels for that power, but allow easier means to manipulate it.

Illegal soul trafficking is big business, after all.

You can also take the Arkay angle, with soul gems being another FU to Arkay from Molag Bal.

Imperial Census of Daedra Lords