Israel is currently blockading Gaza, their occupied territory. All supplies must go through Israeli checkpoints. This is ostensibly to limit Hamas (the rulers of Gaza and sworn enemy of Israel) access to weapons and other material.
Many people feel this is unjust, as it is functionally collective punishment for the people in Gaza. Activists with the Free Gaza movement sent ships with aid supplies directly to Gaza, hoping to break the blockade. They have done so before, and gotten diverted (I believe one instance they were allowed through, but detained later). There were some statements by the activists beforehand that they were trying to provoke a violent response from Israel to get publicity.
Anyway, 70 miles (iirc) from the blockade, in international waters, the Israeli Defense Force (IDF) intercepts the ships and tells them to divert; they refuse. So the IDF, apparently, fires warning shots and starts sending commandos onto the ships from helicopters. This was probably illegal, since the ships were still in international waters. It was also a pretty terribly stupid move from a tactical perspective.
On the main ship, a mob of about 30 activists on board rather savagely attacked the IDF commandos as they landed on deck, with pipes and other makeshift weapons. In response, the IDF soldiers (armed only with paintball guns and sidearms) shot and killed 9 people.
It is unclear who initiated the lethal level of violence. 570 other activists on the boat did not attack the IDF soldiers. Having looked at the videos, it is difficult to conclude that the attackers were acting in “self-defense”; they behaved like a violent mob out for blood. But then, the IDF’s decision to board the ship in international waters in the middle of the night from helicopters was extremely aggressive, and they were poorly prepared to deal with what should have been a predictable mob. Some people have claimed the IDF fired live ammunition before the encounter, but there is no evidence to support this. Israel’s government has claimed the attackers were terrorists or al-Qaeda affiliates, but have provided no evidence to support this either.
The whole thing to me seems like a microcosm of the entire Israeli-Palestine conflict, with Israelis ignoring international law and using disproportionate lethal force, and the other side engaging in stupid and provocative violence—with both sides’ supporters engaging in knee-jerk defenses and casting hyperbolic aspersions.