The guy who made my avatars is following me on tumblr!
Those avatars are amazing!
Re: Terezi, I bet she was serious at the time. I seem to remember Vriska used Flarp to collect trolls for her lusus to eat. Terezi was Vriska's flarp partner. She kills her dolls constantly, it's reasonable to think that she'd view some anonymous alien dumbass the same way.
But then, like Vriska, she discovered that killing someone you know is different. She seemed sorry about killing alt-John after Dave explained that it actually, in a sense, happened. Later she was seriously distraught over alt-Dave's death.
A big part of the troll arcs is watching them try to find their place in the Alternian culture of violence and death. Even Tavros wants to be a cavalreaper (or possibly a military strategist, given the nature of Fiduspawn). Kanaya wants to hunt the undead. Equius constantly fights robots and wishes to join the archers. Almost all of them want to be killers when they grow up. The ones who do commit murder, though, are shocked - murder isn't actually in their blood, but just a culture fostered by Doc Scratch and The Handmaid.
So I think Terezi fully intended to kill John, then *did* (thinking it was impossible, mind) and felt regret afterwards. Regret that made it hard for her to stop Vriska (as she tried to explain to Dave before the confrontation).