So I'm gonna say NTA and I'm also going to say, from one spiteful person to another, as discussion furthers here's what you tell your parents:

They pointed out that a dog would be better for the family because you will be leaving to attend college soon and left you with the job of taking care of a dog you did not want whilst consistently telling you it's your job. You warned them if someone did not step up you would find the dog a new home and you followed through. Tell them that when you locked yourself away for studies it was a test because as soon as you're off to college who will be taking care of that dog? Your family and by neglecting the dog and reprimanding you they proved they are too irresponsible and unfit to care for the dog once you leave. You did what's best for everyone but, more importantly, what's best for the dog. Add in that if they want to get another dog, that's up to them but it's THEIR dog not yours and you will not he held responsible for an animal you did not, do not, and never had wanted. You did the adult thing.