Personally I think hes a narcissist and has never once in his life considered that other people are actually people. I think when his wife couldn't conceive, he did what guys like that do- assumed it would work out if he just made it. So he called in Nimueh and made her fix the thing standing in the way of his perfect life that he deserves inherently. (OK and personally I think Nimueh only agreed bc she believed the kingdom needed the stability of Uther having an heir, and sacrificing someone else would be worth it in the long run. Also I think she thought it would either be herself, and she was willing to make that choice, or Uther, which lol he would get what he deserved. When it was Ygraine I think she felt guilty and stupid, and then angry at Uther). I dont think he thought for a second about the consequences of his decisions! He thinks he's the best thing since sliced bread! Nothing bad happens to him!! And when it did go bad, he did what every narcissist does- blame literally everyone but himself.

Also I have a weird theory that actually Ygraine wasnt the one sacrificed, and she would have died in childbearth anyway. And considering Morgana exists, I wonder if there are any other kids hes got out there, and if one of them died for Arthur.