They appear to have been kidnapped, but there is no proof of that yet. Two were Israelis, and one was an American Jew. The Israelis lived in an illegal colony called a settlement, and the American was visiting the colonists in their colony.
The native Palestinians around Hebron are very militant. There are 500 Jewish colonists living in a city of 90,000 Palestinian natives. The 500 colonists cause incredible disruption to the natives and abuse them constantly. An Israeli military force is stationed there to guard the colonists and they often commit abuses against the natives too. Hamas is known to operate in and around Hebron. Assuming Palestinian groups are behind this, I would suspect Hamas.
The teens were driving in a vehicle when they disappeared. A burned out vehicle, possibly theirs, was found in the vicinity. If they were captured, they were probably taken alive and I doubt if they have been killed or that their captors intend to kill them. They probably want to keep them as bait for a prisoner exchange. If the Israeli military tries to rescue the teens, I would expect that the captors may kill their captives.