Presumably because the bear has free reign to roam wherever he chooses, and as such poses continued risk to the public. The whale certainly cannot swim from home to home, terrorizing children at play.
The only person (people) or animal with any culpability here is the trainer and the management staff of Sea World. If the animal is so unpredictable, that it needs to be isolated, then so be it. But, that's not license to kill it out of pure convenience. Either return it to the wild, or continue to care for it, as is the park's assumed responsibility when it placed it (or it's parents) into captivity.
You consider saving human lives a matter of 'pure convenience'?
You do understand that one of the three dead was not a trainer or on the staff, correct? I had initially made the same point you did in my 'mitigate' comment in my first post, before realizing my mistake of saying only trainers are at risk. While not condoning trespassing in the least, it's clear that trainers are not the only ones at risk if they are not the only ones who have already died.
I believe in liberty - that is, freedom to act in within a moral context. That applies solely to humans who operate within a moral framework. It does not extend to animals in either sense. That is, I don't blame the animal for 'murder' in a human sense, but neither to I say the animal is blameless and that it should be allowed to kill again. The dead victim's liberty has been dramatically curtailed, and that is what matters.