As a matter of fact, I do.

"Reasonably believes" is a reference to our old friend, the "reasonable man". The "reasonable man" is supposed to be an objective standard of reasonableness. In other words, you don't take aspects that are particular to the person in question; rather, you consider a hypothetical "reasonable person" in their place, and try to figure out what they would have done.

In practice, there will typically be two applications of this standard. First, the police will decide whether or not they consider the actions of the perpetrator to have been reasonable. The outcome of that decision will determine whether or not they arrest the perpetrator. Whether or not they decide to arrest him/her, there will still likely be a civil suit for wrongful death, and at that point, it's up to the jury to decide whether or not the (now) defendant acted reasonably.