Was it out of character?

[Mister Underhill]

The analogy is perfectly acceptable.

The analogy isn't even close to being apt. Sauron isn't just some dude that The Aragorn Gang is trying to bushwhack for bragging rights, or a bounty, or because of some personal grudge.

Sauron is the aggressor, the supreme commander in a war that he started, that has already claimed thousands of lives, and which could potentially lead to the enslavement of the entire population of Middle-earth. There's a big difference between the ambush of an individual and a strategy aimed at destroying an aggressive enemy's ability to make war on you.

Say Wyatt Earp rides out to your ranch (unprovoked, of course) with a posse of cut-throats and bushwhackers, sets your house on fire, kicks your dog, steals your wife, and tells you he's gonna raise your kids to be his horse-stall muckers just as soon as he kills you, and then you're starting to get into an analogy that isn't absurd.

One thing that really bothers me about this scene is the way Aragorn rides right up to the Black Gate without drawing a single arrow shot. Seems even Mordor orcs have more respect for a parley than he does...