A person armed with a knife can be on you from that distance in a second or less. You don't know they are having a "mental episode." And, what if you did know that?
They are still a threat. Its not like being in the midst of a mental crisis makes the knife any less dangerous. In fact, arguably, they might be more dangerous because they lack restraint.
Law enforcement officers are trained that, when faced with a person who is armed with a deadly weapon, and who represents a threat, you cannot take chances with less than lethal force because you might not survive it.
Cops are not trained, for example, to shoot them in the leg, or the arm, or anything similar. In hindsight, you can always come up with, what ifs? What if you tried a taser, a stun grenade, called for a counselor, etc. But that is not a realistic view of the situation, on the ground, as this person continues to advance despite being told to stop. Dave is correct.