Why did Tallahassee say, "That decides it"?

In Zombieland Double Tap (2019), after leaving Babylon, Tallahassee came across T800 kind of Zombies:

Tallahassee: T-800s. That decides it.

Best Answer

Tallahassee sees himself as a father figure/mentor/protector to the younger members of his 'crew'. The 'chief' of his 'tribe' so to speak.

After our intrepid heroes locate Little Rock at Babylon, Tallahassee tries to fit in but has a hard time of it. He disagrees whole-heartedly and vocally with the whole 'peace-nik' concept of Babylon and doesn't want to stay. He tries to convince himself that he is leaving Columbus, Wichita, Little Rock and (to a lesser extent) Madison in relative safety at the hippie encampment. I don't think Berkeley matters much.

<Seeing Little Rock safe and surrounded by friends her own age, Tallahassee realized his nest was now empty. And he began to hear the call of the buffalo.>
_{source: Zombieland: Double Tap (2019) transcript}

But leaving them in Babylon is not as easy as he makes it out to be, regardless of how much he hates the place. While it's nice to be back on the road as a 'lone wolf', he can't stop worrying about them. This indecision comes to an abrupt end when he strikes the T-800 on the road away from Babylon.

TALLAHASSEE (singing in the pickup truck): Give me a home, Where the buffalo roam, And the deer and the antelope play. Where seldom is heard, A discouraging word, And the skies are not...
_{source: Zombieland: Double Tap (2019) transcript}

Tallahassee hits something on the road.

TALLAHASSEE: T-800s. That decides it.
_{source: Zombieland: Double Tap (2019) transcript}

It refers to the internal conflict between leaving his crew at Babylon to pursue his own path as a lone wolf and staying somewhere he hates in order to protect and look out for them. After hitting the T-800 on the road and seeing tens if not hundreds more making their way towards Babylon his mind is made up.

He decides on the spot that he will turn around and put up with all the bullshit Babylon has to offer if it means keeping his crew safe. He slams his pickup in reverse long enough to pull off a quick J-Turn maneuver then accelerates quickly back to Babylon to warn (and save) his adopted family.

_{Tallahassee to the rescue.}