Why would you leave your crew in the dark about a potentially fatal scenario just to satisfy your own selfish ego? If he was trying to make a point, why did he keep it to himself until he was confronted with it?
First, it wasnít a potentially fatal scenario. It was a simulation. Cadets were supposed to act in the simulation the way they would if it were real, and in that sense Kirk certainly violated the spirit of the test, but his position was that the test, as implemented by Spock, wasnít realistic, so he didnít.
As for keeping everybody else in the dark, he obviously enjoyed the flair of it, but it was also necessary. If Spock or the other test administrators had known what was going to happen, they would have reversed the tampering (or, if they couldnít fix it in time, they would have aborted the simulation).