I went to answer the doorbell and left my young son alone while he was in the bathtub
You should have taken your son out of the tub before answering the door.
But it was the house next door, they told me their was a fire.
You didn't know that this was an emergency when the door rang, though. It is wrong to leave your kids alone in the bath when they might drown. You should either take them out of the water, or get a video doorbell to talk to the person at the door.
look it's great now. I took him to the hospital, who cares about potential injuries.
I assume that you mean that you got your neighbor to the hospital after the fire, and that is a good thing. However, it is still wrong to leave your children in danger, regardless of the circumstances. In this case, your child was okay, but he might not be so lucky next time.