Tillikum has been in the water with two other deaths; correct. But there is no proof in either of those instances that he killed them. And one of them easily could have been prevented by an intoxicated guest who snuck past park security and striped down to nothing, and jumped in with him. He died of hypothermia.
Working with ANY animal can be a dangerous job. Nearly a year ago, an elephant trainer was killed when the elephant pushed her into a wall. Everyone that works with animals knows it's a potentially dangerous job. Just like every police officer knows their job is also dangerous.
And as far as Tilikum being a "dangerous" animal. What happened has been ruled an accident. SeaWorld has over 5 million documented trainer interactions recorded with Tillikum, and there was only one accident. These trainers are more at risk of getting killed in a car accident on their way to work, than having an accident at work happen.