Can you target your own creature with a spell that says 'destroy target creature an opponent controls'?

A spell or ability can only target objects or players that match its targeting requirements exactly. If a spell reads 'destroy target creature an opponent controls,' the target must be a creature controlled by one of your opponents — full stop. This is governed by CR 601.2c, which states that targets must be chosen according to the specifications on the spell or ability.

Your own creatures are under your control, not an opponent's control, so they simply do not satisfy the targeting requirement. Attempting to declare one of your own creatures as the target would be an illegal target selection, and the game would not allow you to cast the spell that way (CR 601.2c, 608.2b).

This is different from a spell that says 'destroy target creature' with no controller restriction — in that case you could legally target your own creature.

Concrete example: You control a Grizzly Bears. Your opponent controls a Llanowar Elves. You cast Murder (if it said 'destroy target creature an opponent controls'). You must target the Llanowar Elves. You cannot choose your own Grizzly Bears as the target, because your own creatures do not meet the 'an opponent controls' requirement.

Bottom line: always read targeting restrictions carefully. The phrase 'an opponent controls' is a hard restriction that excludes permanents you own or control yourself (CR 109.5, 601.2c).