But TCAS is preempted by GPWS and stall warnings, so the damage is limited to cases where you are flying at altitude directly under another airliner and you can spoof an RA for the plane above to descend into the plane below. The statistical likelihood of this configuration and a malicious attacker that can spoof TCAS is probably so low as to not cause much worry.

(Also, despite procedures, airline pilots are not automatons. They may be able to insert their brain into the loop to avoid disaster, despite the opposite happening from time to time.)

TCAS says "Traffic! Descend!" Pilot descends abruptly, calls ATC, resumes normal navigation. Happens again, same outcome, this time pilot notes TCAS inop, turns it off.

TCAS isn't "completely" automated, as it requires the pilot to actually obey. There's a squishy brain in the loop. Given that, I'm having a hard time imagining a way to do any damage (although you could easily create a huge hassle if you could spoof alerts at will). Even if you could create TCAS alerts that would steer planes into each other, the TCAS would then detect that and separate them again.