The Brave, But Dangerous, Sir Not-Too-Bright

Ah, yes...

Him.
If you've read the TV Tropes on Monty Python and the Holy Grail you might think that Sir Lancelot is an idiot as all the tropes related to him seem to be about his blood-lust.  
http://tvtropes.org/pmwiki/pmwiki.php/Characters/MontyPythonAndTheHolyGrail
Well the truth is that the Pythons were being very clever with their characterizations and really hit Lancelot on the head squarely, but you have to be paying attention to notice it.  And you have to know your background sources.  
Lancelot was given to bouts of madness, usually when treated brusquely by Guinevere or when his guilt got to him and he was so strong of arms that we should not be at all surprised that he solves many problems with violence, but as Chretien de Troyes told us how he shamed himself by riding in a cart to rescue the queen when she was captured, we can see he was not limited to violence to solve all his problems.  This is not a blood thirsty idiot we are talking about.  Even in the Python film it should be noted that when Arthur sends Lancelot to answer the questions 3 at the bridge of death his immediate response is as follows:
"Yes, let me go, my liege.  I will take him single-handed.  I shall make a feint to the north-east..."
See the subtlety there?  The man is not running straight into battle like a berserker.  He knows his tactics.  And let us not forget that he was the first to successfully answer the questions 3.  Okay, they were pretty easy, but my point is made.
Now to my larger point: YOUR FIGHTER DOES NOT HAVE TO BE STUPID.
I have made a career in RPG gaming of playing Fighters that avoid fighting, not because they are cowards but because they want the tactical advantage before they risk their precious hit points.  Back in the day when we played in small groups we almost never had a Cleric and the time we did he was an evil Cleric of Loki.  Heals were not forthcoming and we didn't have bottles of magic Tussin strung to our belts like a game of Diablo.  
We had to pick our battles carefully and carefully we did.  If I could talk or intimidate my way out of trouble I would, every time.  And when I couldn't, well there is much more to fighting the foe than running straight at him screaming.  I've talked down DRAGONS, mate.  Intelligent foes can be reasoned with...or at least distracted long enough for the nukers to get into position.  
So you've rolled up your new Knight, Sir Abattoir and he has a decent intelligence score and it's time (3rd level) to pick your Martial Archetype.  Consider Battle Master.

IRONCLAD-James Purefoy defends a keep with a handful of soldiers against the might of the King's army.  Be this guy...

What does Battle Master bring to the tabletop?
Maneuvers, that's what.  The Battle Master foregoes personal improvement in raw damage output and self-buffs for party-wide tactical skills.  The Battle Master is a leader, an officer, a KNIGHT.  He controls the battlefield and organizes his allies.  This is Aragorn, this is Odysseus, this is Jason, this is the Glory of Rome.  You remember Gladiator?  Maximus is a Battle Master, a general, a student of the tactical art of war!  
These maneuvers include goading the enemy into making mistakes, directing your allies to strike vital points, rallying your comrades, and much more.  You gain Superiority Dice which you hand out with your maneuvers to increase attack and damage rolls.  You gain access to tools (useful for making siege engines or alchemical weapons) and learn to observe your enemies to find their weaknesses and exploit them.  

... NOT THESE GUYS!

This is not some blood thirsty crazed idiot with a weapon.  This is a tactically superior merchant of death, selling his resources dearly, not cheaply.  It is the essence of the heroic warrior, really.
If you are going to go the Battle Master route try to keep it in mind from the earliest levels (1 & 2) even before you gain the archetype.  Don't rush into battle; make the enemy rush and then trip him into the spike pit you dug before the battle even started.  Use the correct tool for the job.  Don't be afraid to tactically retreat and regroup and always remember to FIGHT SMART.  Remember Oct 25th, 1415, when Henry V won the Battle of Agincourt against a numerically and technologically superior French force by using every sneaky tactic at his disposal.  

Did NOT happen at Agincourt...but it would have been bitchin' if it did.

Of course he didn't have dragons and beholders to contend with either.  Which is all the more reason that you, the Battle Master, need to fight smarter not harder. 
Don't let the so-called smart characters (you know, the Wizards) do all the thinking.  Your Fighter can be both smart and kick all manner of ass and you will enjoy the game all the more if he does.