“Nothing outside a person can defile them by going into them. Rather, it is what comes out of a person that defiles them” (Mark 7:15).
In Mark 7, Jesus did not say every act is justified as long as the dead bodies that result from the act are eaten. Instead what he said is that what comes out of a person is what defiles them. He was responding to an accusation against his apostles that the bread they were eating is unclean because they did not wash their hands first (Mark 7:1-5).
There were still plenty of food prohibitions among his followers after Jesus made this statement. For instance, in Acts 15:29 Paul says to stay away from the meat of strangled animals, from blood, and from food offered to idols. And nobody (I hope) is trying to justify cannibalism based on what Jesus said in Mark 7. So clearly it is not referring to all things that can be eaten, and certainly not permission to murder a person to eat their dead body. So how then can it be reasonably interpreted to mean Jesus was changing the law to, for instance, permit killing pigs to eat their dead bodies?
Like most other passages that are taken out of context and misinterpreted to justify violence and serving the desires of the flesh (Philippians 3:19), this passage has nothing to do with Jesus permitting all forms of animal cruelty or saying anything about what should or should not be eaten. Instead it has to do with the man-made tradition of washing your hands before you eat and he used the opportunity to teach about the importance of having our heart set first and foremost on following God rather than man. That’s what Mark 7 is about.