You are not a terrible person, you have to go with your heart
I knew there would be problems when I introduced another dog and it was a nightmare, Ben hated Harry on sight, he is not a sociable dog anyway, and they spent a lot of time separated by a baby gate. We had lots of growling and snarling and a lot of nasty spats and I spent the first few weeks wondering what I had done and feeling guilty for turning Bens world upside down, because I wanted a second dog. Harry was a rescue and had his own issues which didn't help. I had excellent advice from Top Barks and support from Col members and we got through it. There are ways to cope with two dogs who don't like each other much but from my experience it took weeks and weeks before things settled a bit. Then probably another year
before I relaxed and started to enjoy them both.
You have to go with what you feel is best for you and your family. We have no children to worry about when there was growling etc, so it made life easier and I guess I knew it would be difficult so it wasn't a complete shock., but I honestly don't know if I would go through it again, the first few months were more stressful than enjoyable. If you feel Smithy has to go back then don't feel bad, as you have to do the best for you all, but if you do want to keep him there are ways, but it is a difficult road and takes a lot of time.