Why was the actor playing the director in the Truman Show willing to
kill the main character Jim Carrey was playing- this didn't make
complete sense with the idea that this movie was representing the way
the media affects us today- the director guy was willing to put up the
notch of water storm into danger area even though everybody around him
was protesting, and they just sat by and let him do it.

Here's my view of the matter:

The director, Cristof, is more interested in creating a dramatic
situation to entertain the viewers than he is in preserving human
life. He sees Truman more as a prop than as a human being. From
Christof's standpoint, Truman exists only for the purpose of the show.

A network official says "For God's sake, Chris, the whole world is
watching! We can't let him die in front of a live audience!"

And Christof's response is "He was born in front of a live audience."

It is perfectly appropriate - they'll just get a new Truman. The media
always needs something new.

Have a good day.

I like Pink's answer, and I also think that there was some parental
instinct involved.

Christof had always thought of himself as Truman's protector and in
many ways his parent.  Parents are often overprotective... even to the
point of harming their child.  For instance, if your child was about
to run into a road without looking both ways, would you be willing to
quickly grab his arm firmly as he started to run?  Sure you'd probably
bruise his arm, but you'd save him from the possibility of a far worse
fate.
The far worse fate in this case is the cruel world outside of the
world Truman knows.  For Truman to see this world and live in it could
be a fate worse than death in Cristof's mind.  Cristof did everything
in his power to give Truman a good and safe life far away from the
influences of this horrible world.

The actor playing the director in the Truman Show did this because
that is what his job is - to act.

The actors in a movie are not responsible for the story line/script.

Cheers

I have always viewed it that Christof has gone off the deep end and believes
he is the god for the Truman "world" and as such, and as he (Christof) was the
creator of Truman, he certainly had the power to test Truman's mettle.

If Truman died through that process, Christof believed that was all within his
power, and the people around him were willing to let him assume that power.