If you're referring to Tilikum, he most certainly does not deserve to be killed. If anything, he deserves to be released. He killed three people because he has been psychologically damaged by years of captivity. SeaWorld is a fool to keep him after the death of Dawn Brancheau. It's SeaWorld's fault, not his.
What are you expecting them to do? We know that their trainers were innocent but they obviously dont. Being held captive to amuse humans, separated from families, taken away from their home, its no wonder they become against they're trainer. They dont belong in a zoo, they should be out in the wild living their life.
Animals do not belong in captivity. Period. They are only kept for our amusement and profit. In zoos, animals show signs of depression and zoochosis. Tillikum the orca at sea world has been in captivity for 30 years after being torn away from his family when he was just 2. He has killed 3 trainers during shows and is now used as a breeding tool -kept in solitary confinement in a pool equivilant to the size of a bath tub- Though dont you think you would go crazy after 30 years of swimming in circles? It has been said that Tillikum knew what he was doing. He had studied the humans and knew that when he dragged his trainer under water, she would drown just as orcas and dolphins in captivity know that continuasly bashing its head against the tank wall will kill it, how they know that by closing their breathing hole underwater will drown themselves. It is VERY common for ceteceans to commit suicide in captivity. So no, animals that kill their trainer should not be killed. They should be released into the wild where they belong. Not in chlorine pools doing tricks for screaming children. Its the humans fault, not theirs.