TG: why not let her go get her shit ruined by jack and let justice happen that way
GC: B3C4US3 1 4M QU1T3 SUR3 TH4T 1F SH3 GO3S TO F1ND H1M, 1T W1LL T1P H1M OFF TO OUR LOC4T1ON 1N TH3 V31L
GC: 1 H4V3 S33N 1T 4LR34DY
Either Vriska dies, or everyone dies - and this isn't baseless speculation, either. It's coming from a Seer.
Terezi essentially has no choice but to act. When she declares that personal feelings 'don't matter' to a legislacerator, it's not supposed to be a boast - quite the opposite, in fact. It's something she's saying to herself, in an attempt to suppress her reluctance to kill Vriska.
GC: BR1NG1NG H3R TO JUST1C3 1S CR1T1C4L TO OUR SURV1V4L!
TG: so why dont you go do it […]
GC: 1M NOT SUR3 1F 1 C4N
Terezi is completely backed into a corner, here. She's twisting herself in a circle, desperately trying to find any excuse not to kill her frenemy...
...and it's not the first time, either.
The universe just keeps forcing these girls into impossible situations, where they feel like they have to make each other pay. Talk about star-crossed haters!
TG: you mean you cant beat her in a fight
GC: NO, 1TS NOT TH4T
GC: 1T JUST TH4T WH3N TH3 T1M3 COM3S
GC: 1M NOT SUR3 1F 1 W1LL B3 4BL3 TO K1LL H3R
I also think Terezi's getting a little less ok with murder in general.
When Green Dave succumbed to his inevitable doom, her cheerful veneer was temporarily shattered. Against her better judgement, Terezi Pyrope is actually starting to value a life.
...and, just as she's discovering this about herself, she also learns that Vriska needs to die, for the good of the Veil. How unlucky do you have to be, to have both of those realizations simultaneously?
GC: W3 4R3 SUPPOS3D TO R3V3L 1N BLOODSH3D 4S W3 GROW UP
GC: 4ND SH3 S33MS TO B3 3MBR4C1NG H3R R1T3 OF P4SS4G3 W1TH R3CKL3SS 4B4NDON […]
GC: 1TS 4 L1TTL3 1NT1M1D4T1NG
GC: B3C4US3 1M NOT SUR3 1F 1M R34DY FOR TH4T […]
GC: 1 4M NOT SO MUCH WORR13D 4BOUT NOT B31NG R34DY
GC: 4S 1 4M TH4T…
GC: 1 M1GHT NOT 4CTU4LLY W4NT TO B3 R34DY
GC: M4YB3 3V3R
GC: M4YB3 TH3R3S SOM3TH1NG WRONG W1TH M3
This epiphany is probably a lot more common than you'd think.
I firmly believe that trolls aren't murderous by nature. They're just taught to be, by the Empire that shapes their lives. This is why contact with humanity is fucking them up so badly - it's the first time they've been confronted with the idea that life has value. They're learning about a world where they'd be considered evil for what they do.
I feel genuinely bad for Terezi, here. She's beginning to realize, with dawning horror, that what she was doing was wrong.