When you do some digging around on the internet you'll find some stories from people who were around when John Palmer was at the height of his power. His Tenerife exploits are legendary - I remember I read someone stating that Palmer practically owned that island back in the 90's lmao. He made way for quite a few notorious firms based in East/Southeast London, Essex and Kent to operate rackets in Tenerife.
He had an issue with a certain Hezbollah-connected Lebanese clan after one of his roided up coked up henchmen blinded one of the Lebanese. The Lebanese flew in a small army of family members, but after a few fights they got chased off.
Later on he allegedly fucked over a Russian gang as well who cooperated with him on one of his timeshare scams.

Between London firms, a Hezbollah-linked clan, Russian gangsters and others there must've been at least a hundred people that had good reason to take him out.