There are some frighting scenes but not too scary
Shocking for the time the show wasn't afraid to kill to kill off main characters this became a staple of the star trek series.
Tense arguments about command and political strive.
Kirk has anger problems and often smashes things etc this can be frightening due to the realistic nature of these scenes.
Often times Phasers are set to stun and not kill but sometimes people are killed and it can often be sudden and on screen especially in the final season.
The OST can be threatening sometimes especially during the infamous "You can't kill me, I'm part of you scene." from season 1.
The show doesn't deal in good and evil and is morally complex although Kirk is seen as the good guy there is no clear cut good person.
Some decisions led to massacre and war.
Characters freeze to death, in one scene a main character does so and it is upsetting, this was especially shocking as it was the episode before the end of season 1 and no one at the time expected it.
Kirk has mental health issues that decorate over the series.
The characters' lives are frequently in danger, and some characters DO die.