To put it in other words:
If the gun owner is a great gun owner who does not make any mistakes, the gun will never kill someone innocent.
If the pitbull owner is a great pitbull owner who does not make any mistakes, the pitbull might still get triggered (as they were bred for a strong kill drive) or if you don't believe in pits snapping, might still just straight up have brain problems like any being with a brain might have, escape one day, and kill someone.
That's something people fail to understand. Yes, owners ruin dogs. But also, dogs are living beings with their own complex neural systems and they will sometimes act on their own volition, be it because they were bred that way, or because they have mental illnesses.
In the same way you don't let people own bears in their backyard because the bear might break out and end up killing someone, you shouldn't let people own dogs powerful enough to kill people in their backyards.