Fear Leads to Anger
Anger Leads to Hate
Hate Leads to Suffering
Yoda said those words, telling Luke of the dark side of the Force. And true they were, for a Sith gains much power from raw emotions like anger and hate. Anakin Skywalker was famously (to us at least) afraid of losing Padme, so he did all he could to gain power, so that he might keep her by his side. Then she died, and the Emperor, in a psychopathic-but-entirely-okay-if-you’re-a-Sith move, convinced Anakin that he had killed her, thus finalizing the birth of Darth Vader.
Now, when most folks think of the birth of Darth Vader, they think of the scene in Episode III where Anakin chops off poor Mace Windu’s hand and watches him get tossed out a window. That’s the scene where Palpatine first calls him Lord Vader, and where he throws in his lot with the Sith. However, it could (and, spoiler alert, will) be argued that the true birth of Vader was after he went in for my-ex-bestie-chopped-my-limbs-off surgery. After his suit is fitted onto him, Emperor Palpatine informs Vader that he was responsible for killing Padme. Vader takes this to heart, believing it to be true.
That’s where the temper tantrum part comes in. Vader then proceeds to Force Smash everything in the room: medical droids, little glass beakers that explode in really satisfying ways, the walls, everything. It’s that temper tantrum, that wanton destruction, that truly cements his place as a Sith. A Sith needs anger like that; it fuels them and makes them strong.
Kylo Ren, like his grandpappy* (don’t tell his Force ghost we called him that), has a few anger issues of his own. We all remember that scene in Episode VII where dear Kylo practices his Slashy Form of lightsaber handling on a few totally-not-important consoles. A few stormtroopers on patrol wisely turn around, which just goes to show that even stormtroopers don’t miss everything.
These temper tantrums are important to a SIth. Without them, they’re just people with lightsabers. Normally, temper tantrums are bad, and are discouraged. Not for a Sith. Don’t worry when a Sith has a temper tantrum (just go the other way, quickly). Worry when the temper tantrum stops, because that means the Sith has harnessed that rage, and are ready to use it to do mean things.
*Author’s Note: Calling any Sith a grandpappy is a sure way to trigger a temper tantrum. Don’t do it.