It is not that clear cut Jim.

Piracy?
This is what BBC says.

Israel breach international law?
This is disputed. A Turkish draft resolution circulated at the UN Security Council described the attack as a violation of international law. Turkish foreign minister Ahmet Davutoglu called the raid "tantamount to banditry and piracy" and "murder conducted by a state".
Israel's foreign ministry says that under international maritime law, when a maritime blockade is in effect, no boats can enter the blockaded area. It adds: "Any vessel that violates or attempts to violate a maritime blockade may be captured or even attacked under international law."
The question may hinge on whether the blockade itself is legal.

Did they start killing people Jim?
There is no evidence they did, and considerable evidence that they had no intention of using lethal force, but were themselves subject to attack.