There was absolutely no evidence that it was sentient, or even capable of communication.
The difference is, the Horta showed evidence of sentience ("NO KILL I"). This creature did not.
Good.
Being able to change direction is not evidence of sentience.
I think people get a little too caught up in the sport of moral and ethical debate when it comes to Star Trek, because that's what Star Trek claims to do so well.
Ultimately, though...TOS often found Kirk very
consistently and decisively working to eliminate any threats to the broader Federation population or his crew. They worked to destroy the Deneva parasite creatures, who were obviously sentient and deadly. They killed the M-113 Salt Vampire, even though it was the last of its kind. They pursued the Gorn and Romulan ships who had attacked Federation outposts, with the intent to destroy them.
TOS wasn't playing morality games like TNG may have. Kirk wasn't going to have a drink of tea and think deeply about the morality of wiping out a lifeform that, if you failed to do so, might turn around and wipe you out instead.
Frankly, the Kirk approach is the more realistic and pragmatic approach. He often showed mercy to kindred rivals and opponents on the field of battle, but if he felt you were an existential threat to the crew or (certainly) the galaxy at large...there was no screwing around. It was "
act and act decisively to save lives." That's it. It's one of the reasons I favor Kirk as a captain above the others. I just don't think it's realistic, despite all the blathering done by characters in the franchise about "evolved human sensibilities," to think about the rights of a lifeform to exist when that lifeform has demonstrated the willingness and ability to fuck your shit up. No sir, arm phasers and photon torpedoes and let's make sure we take our opportunity RIGHT NOW to protect the crew and the lives of Federation Citizens. Let history judge that action later. But, as a starship captain, you ACT.
I think fans (as usual) get too caught up in the Preachy Trek myth about how wonderful and enlightened everyone will be in the future and love to pick these kinds of things apart. When, in reality, it's so grossly unrealistic to think that any other choice would be acceptable.
#NecheyevWasRight
#ImWithBooker