Having wasabi sprayed in the face to alert deaf people to fire dangers is infinitely preferable to by injured or even killed as a result of a fire. Because deaf people can't hear normal fire alarms, there has to be an effective way to warn them. Some fire alarms for deaf people involve brightly flashing lights, but they have been found to ineffective because not everyone sees them when they're asleep.
While conventional fire alarms work well for people without a hearing impairment, the deaf have a disadvantage. Without the ability to hear the warning of a fire the hearing impaired must rely on a flashing light to be alerted to a fire. However, there are circumstances where a deaf person would not see this light, such as being asleep. In this case, having wasabi sprayed in the air could save a deaf persons life because it would make use of their sense of smell rather then hearing or vision.
Spraying wasabi in deaf people's faces is grounds for an assault charge and is an obnoxious, over dramatic solution to an easily solvable problem. Most deaf people retain their vision, and thus visual warnings can be provided if needed, like holding up a sign or flashing a light. You can tap a deaf person on the shoulder or pull them out of danger if necessary without any need for spraying something spicy in their faces. Additionally, if there really is no other option but to spray something in a deaf person's face to "save their life," then there's no reason that water wouldn't work just fine.
No, spraying wasabi on a deaf person's face is not justified. It is my belief that wasabi is unnecessary and does not save lives. Spraying wasabi in anyone's face is unnecessary and harmful. Additionally, it is not the case that being deaf means your life is at risk. There are other acceptable ways to alert a deaf person that their lives are in danger.