violence in response to a threat is not legally self defense. If someone says "I'm gonna cut your throat bitch" and you shoot them--you still commited murder. Until they come at you with a weapon or start actually touching you, you have no legal right to use a weapon. Self-defense doesn't just mean "I felt threatened", it means you had no other options and had to use violence to save your own (or someone elses) life. ceist would've been convicted of murder because he wanted to look macho.
it's a little different in 'stand your ground' states--but not until he's actually breaking into your apartment, which he wasn't.
Exactly. Which is why it was a good idea that Ceist didn't actually go through with it, since he would be on-the-hook for manslaughter at bare minimum.