Does Karmic Justice Trigger if a Planeswalker dies from combat damage?
No, Combat damage is not &quot;a spell or ability an opponent controls&quot;.
It's not a Spell:

112.1. A spell is a card on the stack. As the first step of being cast (see rule 601, “Casting Spells”), the card becomes a spell and is moved to the top of the stack from the zone it was in, which is usually its owner’s hand. (See rule 405, “Stack.”) A spell remains on the stack as a spell until it resolves (see rule 608, “Resolving Spells and Abilities”), is countered (see rule 701.5), or otherwise leaves the stack. For more information, see section 6, “Spells, Abilities, and Effects.”
112.1a A copy of a spell is also a spell, even if it has no card associated with it. See rule 706.10.
112.1b Some effects allow a player to cast a copy of a card; if the player does, that copy is a spell as well. See rule 706.12.

It's not an Ability:

113.3. There are four general categories of abilities:
113.3a Spell abilities are abilities that are followed as instructions while an instant or sorcery spell is resolving. Any text on an instant or sorcery spell is a spell ability unless it’s an activated ability, a triggered ability, or a static ability that fits the criteria described in rule 113.6.
113.3b Activated abilities have a cost and an effect. They are written as “[Cost]: [Effect.] [Activation instructions (if any).]” A player may activate such an ability whenever they have priority. Doing so puts it on the stack, where it remains until it’s countered, it resolves, or it otherwise leaves the stack. See rule 602, “Activating Activated Abilities.”
113.3c Triggered abilities have a trigger condition and an effect. They are written as “[Trigger condition], [effect],” and include (and usually begin with) the word “when,” “whenever,” or “at.” Whenever the trigger event occurs, the ability is put on the stack the next time a player would receive priority and stays there until it’s countered, it resolves, or it otherwise leaves the stack. See rule 603, “Handling Triggered Abilities.”
113.3d Static abilities are written as statements. They’re simply true. Static abilities create continuous effects which are active while the permanent with the ability is on the battlefield and has the ability, or while the object with the ability is in the appropriate zone. See rule 604, “Handling Static Abilities.”