“how would you commit a crime... alone on a desert island?”
Trespassing if the desert island is private property.
Illegal immigration if the desert island is within the territory of some nation.
And of course murder, if you murdered everyone who arrived on the desert island with you and that’s why you’re alone.
the ‘alone on a desert island’ hypothetical means you’re the only person around. no one owns the property and the assumption was you didn’t kill everyone.. you spontaneously appeared fully grown on the island on a barren planet