Driving past a no-trespassing sign and through a gate (even though it is open), isn't necessarily 'no big deal'.
You're not supposed to be there, don't go there.
Should you shoot first and ask questions later?
Absolutely not, but if I were the property owner, I'm certainly not confronting the person without taking steps to protect myself, or calling the police, although in Chris's case, that help is far away and probably busy anyways.
At minimum it is a stellar lack of judgement on the driver's part. Private property rights are still a thing in most of the US.