I am playing a level 8 Battle Master Fighter. His world has gone to pot and his entire family has been killed all within a month and a half. So he doesn't give a flying flip about anything but revenge on those who killed his family. He is at max strength and has over 13 in both int and cha. What would be the best way to make him a warlock?

Well, the obvious choice is Hexblade. Even for a single level dip, it's a large boon to get Hexblade's Curse. For a bonus action you get bonus to all damage you output equal to your proficiency bonus (so it still improves when you level up as a fighter). Increases your crit range, which pairs great with Battlemaster Maneuvers to do some crit fishing. Also provides some minor healing, but unless you plan to focus mostly on Warlock going forward, it's never gonna be a game changer or anything. In order to become one you basically just have to find a magic sword at some point.

I’d also say that with a 13 cha, you would not be very good at warlock attacks. So I’d recommend taking spells and invocations that don’t involve attack rolls or saves. Maybe skip eldritch blast, for example. Though you might get some mileage out of booming blade. Focus instead on utility powers and things to buff yourself. Spells like armor of agathys or expeditious retreat. Invocations like devils sight, beast speech, fiendish vigor etc.

I largely disagree with the above. There are a couple things to consider :

1) is your Battlemaster a melee-fighter or a ranged fighter? 

2) does your Battlemaster already have something to do with their bonus action (e.g. two-weapon fighting, PAM, XbowXpert, using certain maneuvers)?

3) does your Battlemaster already have something to do with their Reaction (e.g. Sentinel, Opportunity Attacks, Interception/Protection fighting styles)?

Depending on the answers above different parts of warlock might be useful.

What can Warlock do for you:

1. Area of Effect - Fighters are generally already strong against single-targets but lack options for dealing with multiple enemies. Fiend Warlock can add a few tools for dealing with crowds - Burning Hands, Shatter, Fireball.

2. Mobility - If you are a melee-Fighter and having trouble getting to the enemies to make your melee attacks or if you are a ranged-Fighter and constantly getting ganged up upon, then you can grab Expeditious Retreat, Misty Step, and Thunderstep from Warlock to always be able to get where you want to be. I'd recommend Genie in this case for their flight ability at 6th level.

3. Moar Damage! - Hex is a fantastic addition to the Fighter's arsenal adding a 1d6 damage to every hit, however it does cost a bonus action. Hexblade's Curse is decent as well (single-target and a bonus action), as it Genie's Wrath (no action cost). 

4. Never Die! - Armour of Agathys, Shield (from Hexblade), and Gift of the Ever-Living Ones will keep you fighting all day long.

Because of your ability Scores, multiclassing with warlock is not a good idea for the following reasons:

1. They use charisma for their weapon attacks, so strength is actually useless
2. Charisma controls everything. Their spells, their weapons, their utility. Everything.

If you really want to then talk to your DM. He could maybe organise an event that physically changes your personality in the story, allowing you to become a better warlock.

Thank You for your advice. I have a lot to think about. 

At this stage the usefulness is limited by attribute choices. But a dip could be very much useful for the reasons pointed out by others. 

I wouldn't go very far into warlock. You only need the patron to build the RP theme you are looking for, so lvl 1 is enough. 

You can always link the future fighter progress into your patron RP-wise. Why do they become such a supernaturally strong fighter? The Hexblade boosts them! 

That said, I have to make a sidenote. 

I really love paladin/fighter + hexblade combo for RP reasons. It's of course a very good build for SAD optimization. You get to use charisma for attacks, so you have room for many attributes. 

But combat optimization aside, this is a great RP optimization tool as well. 

I generally dislike MAD classes, because I have little room to get the stats I want for the RP concept, and I feel pressured to have a high str etc. 

A hexadin's statblock could have 14 str and 14 dex, which to me is more fitting for a fighter than 20 str and 8 dex (plate). 😄

Honestly, with these stats if you're just looking for a thematic vibe/story beat, I'd say just try to sit down and work something out with your DM. Even major facets of a character's story arc don't necessarily need to be embodied in a multiclass, particularly when you're gonna be reaching to get much performance out of it.

Bummer posts aside... you must take something into consideration:

- RP wise Warlocks are defined by their contract with a Patron, which can take any way or form (Just talk it with your DM so he can prepare that part for you, specially since you select your patron at 1st level.)

- Mechanically, Agilemind's post covers a lot of the important decisions you should make about what you could take.

Warlocks mainly give you access to spells and Invocations, and with 13 in Cha (and probably needing to wait 4-5 levels for the next ASI to bump it to 14), your save DC and attack bonus with spells will be below average for your level, but that doesnt mean you cannot take utility or defensive options (hell, you can take Eldritch blast if you need a ranged attack option or other ranged cantrip, if is better than a bow is valid when it presents itself) or whatever you think you or your party needs.

- Reading the story you could also take levels in Paladin into Oath of Vengeance, it still takes the same donsiderations about spells with low Charisma, but you have more support/defensive options (or bless), and could just smite people with them....