When I see a laser, I don't immediately think "someone who has a laser sight equipped to a gun is trying to kill me." I think, some dumb teen is fooling around. You could say anytime someone sees a flashlight, they might think its a gun light and start shooting away.
No responsible carrier would open fire on a target who he didn't identify as a threat.
Generally agreed, but be careful with blanket statements. The average person doesn't make a connection between a flashlight and a gun, but for a lot of us, that bright red (or green) dot on my body just screams "gun!". That in itself wouldn't prompt me to fire, but it definitely would kick my alert level up a notch or two.
You also have to take into account other circumstances. Is there enough light to see whether it's a laser on a gun or just a laser? Are the actor's actions in shining it aggressive in other ways? And the list goes on.
Other circumstances. A few years ago, one of my officers shot a guy pointing a cell phone at him. Both Internal Affairs and the Prosecutor cleared him on it. In itself, the shooting sounds bad, but the man was fleeing after committing a robbery, it was almost dark, and he turned and screamed "I'll &#%$ kill you pig!" as he pointed the cell phone. Turns out he'd just gotten out of prison and preferred "suicide by cop" rather than return to prison.
All in all, I think most of us would interpret being targeted by a laser as an aggressive action. Those other circumstances would determine whether or not deadly force is justified, even if, in the end, it turned out to be only a toy.
That would most certainly be a tragedy, but the real guideline is, can you articulate a genuine and reasonable fear for your life?