I cannot find the light switch fast enough befor he get strangled.

Then your PC is too fast, I'm afraid?
I had no trouble finding the switch in time, probably because I activated it during the second day.

Uhm... take a look here, because if teh game runs way too fast, you'll get another problem on day six:
http://www.gog.com/en/forum/gabriel_knight_series/gk1_game_breaking_bug_in_police_station

Follow the instructions posted by pykman.
(Sadly, I don't have the game installed at the moment and can't give you a screenshot.)