I think that violence against animals is wrong, and violence against people is wrong. As is the use of violence against people by animals.
To say that one would justify the other is wrong, and hopefully the environmentalists would see this as true as well.
The highest form of change is non-violent resistance. It doesn't breed hatred and animosity, and though in some cases this would involve trespassing into the area where abuse is taking place, I would find this justified.
One very effective non-violent deterrant to animal abuse is to boycott the place where this is taking effect. And let people know why you are boycotting, be specific and accurate. Money talks.
Having said that however, I think it would be good for the animal rights activists to realize that a lot of the animals in captivity are rescued animals, who would not have had a chance to live out as long of lives without the care of their captors, if at all, and that any resulting offspring are often co-raised by the mother and the staff, and are thus unprepared for a life in the wild.
Animals in captivity are of a great benefit to those who get to encounter them. They educate the public about their habitats, their behavior and the threats that face them. Without them, people would not experience them, the creatures we share this earth with and contribute to the world as we know it.
Animals in captivity are often trained in a style called positive reinforcement. Behaviors that are encouraged are rewarded and behaviors that are undesireable are ignored. This is a very effective system of behavior modification that assists veterinary staff to monitor the health of the animals, as well as feed them safely and appropriately according to their needs.
The abuse of these animals would serve no purpose.
I do not know about how things for animals in captivity are, in other countries, but I've heard of the horrible treatment of elephants, baby harp seals, apes and chimpanzees.
It does merit reformation for sure, but violence is not the way to go.