Electric fence is not cruel. It's very effective. Though depending a situation, there can be problems with the fence getting damaged,shorted,and you have to stay on top of checking and mending, even doing daily patrol to brush all the crispy fried ants out of the insulators that have shorted it out, lol. and having to do that with a mile of fence run on even a small farmstead can be a duanting and time-consuming task!
I used to instruct dog obedience training classes for pet owners. That 'poor baby, but I can't DO that to him,' owners that didn't want to give corrections fast and hard and consistently to get results, always had to be dealt with right off the bat. Part of my first session lecture was to the effect that the most cruel thing you can do to your pet is to fail to administer correction with enough force to get it across to the dog, so that it gorws into anunruly monster no bodt can live with. Where the dog already had behavior problems,which with pet obedience classes, there ware always plenty of those, the lecture was even stronger....you are here because you can't live with this any longer....well, know that no one else will either, and either you do what it takes to correct the dog, or don't waste my time and yours....just go let the vet humanely euthanize it now.
If your electric fence not only keeps that dog out of your chickens, but may even TEACH it to leave chickens alone, you've done the kindest most humane thing possible for that dog.