Most of the humans and other aliens there could also kill everyone there, if they wanted to - e.g. Capt. Picard, or Geordi La Forge, or Beverly Crusher. Data is a bit of an exception in that without anyone else around, he could still fly the ship on his lonesome. He could also just float around in the cold empty wastes of space for a few hundred years first, to avoid being caught by any authorities.

What stopped him wasn't some "override" - or if one existed, he demonstrated multiple times how he could also override the override - but the simple and plain fact that he did not want to. They knew him, they trusted him, they understood his motivations. And, e.g. if he ever did kill everyone, his career in Starfleet would most definitely be over. Plus, they were his friends. That is a powerful blocker:-).