Why do they gotta turn the plane around?

FTA:

However, moments before departure, the flight's crew discovered that the woman had died. The aircraft was forced to return before it had even departed the tarmac, with the journey from Spain to Gatwick eventually postponed by 12 hours. Travellers claimed the dead woman had been taken to towards the rear of the aircraft in a wheelchair before she was hoisted into her seat with assistance from five family members.

so. first off, the plane had never taken off by the time they had found her dead.

Even then, the flight was from Spain to the UK, so jurisdiction matters. if she died on the plane (which is what Easyjet says) she still died in spanish territory. And if she died before they boarded... well... that's probably something they want to look into.