This has been done before with Xalos's Murder Mystery where if you look at a player it will give you his status.
So it is possible.
I actually made a system called the Gaze Mod which does literally exactly this. It has onGaze, onGazeEnter, and onGazeLeave. It was rejected by Ephialtes due to looped raycasting. If anyone wants, I could put it up in the Add-Ons section.
EDIT:No.
Give the event a distance limit. You have to be a certain distance near it for it to work.
This way, its no different than onPlayTouch > Kill.
My mod actually has this. The default range is either 20 or 8 TU (40 or 16 block studs respectively), and can be set in RTB prefs.