It's not broken at all (following is all based on 2014 rules). So you get temporary hit points equal to your Warlock level + Charisma whenever you reduce a creature to 0 hit points.
Let's take a look at level 3. We'll assume a Charisma of 16. Your best damaging spell is probably Scorching Ray (granted by Pact of the Fiend), which gives you three attacks for 2d6 fire each. If you have Hex up, you'll get another d6 necrotic on each attack. If you hit with all three attacks, that's 9d6 damage, or 31.5 damage. Pretty sweet. But, let's look at the enemies you might be fighting.
A high difficulty encounter for a level 3 party can be either a CR 3 Ankylosaurus (68 hit points), 2 x Adult Kruthik's (39 hit points each), or 4 x Bugbear (27 hit points each).
Fighting the Bugbears, your Scorching Ray obliterates one of them no problem. You get 6 temporary hit points. Each Bugbear can attack for 11 with their Morningstar, though if they surprised you, you might have already taken 18 from a single Bugbear (ouch!). You have two Pact slots, so it's possible to kill two Bugbears and get 12 temporary hit points from this fight, about enough to ignore a single attack (and hopefully you have a short rest afterwards!).
Fighting the Kruthik's, you need a better than average roll to kill one outright to get your 6 temporary hp. It's not inconceivable that an ally can hit one first to guarantee the kill, but depending on initiative, you might deal most of the damage and someone else steals the kill from you. The fact that you can split up your rays might make it possible to kill two birds in one action, depending on what's going on, but you can only get the 6 temporary hit points once. By contrast, each Kruthik has two attacks that can deal 13 damage, so again, the temps are only equal about one of it's attacks, half the damage it can deal per turn.
Fighting the Ankylosaurus, you're not guaranteed the kill. Two scorching rays alone won't kill it, but most likely your other party members will take it out before you do. It's unlikely you get any temporary hit points. Even if you did, if you were hit by the Ankylosaurus, it deals a whopping 18 damage per hit, so you'd only shave off 1/3 of a hit.
Because of how monster hit points and damage scale, things just get worse from here. Monsters become harder to kill, and deal more damage, and the scaling of your temporary hit points isn't great. The advantage is that you can carry them over into future battles, but it really is a drop in the bucket. One would hope, as a ranged character, you wouldn't be taking that many attacks anyways, which actually devalues the temporary hit points if they're not being used.
As for the 2024 Fiend Pact, you get the temps no matter who kills an enemy, which is good, but it requires you to be within 10 ft. of that enemy, which is way too close for comfort. The scaling is still bad, and even worse, the 2024 guidelines for encounter design will have you facing more or even more dangerous foes. For example, while that 2 x adult kruthik encounter is hard in 2014, in 2024 it becomes merely moderate- you'd have to face 3 adult kruthik's to get a high difficulty encounter!
So while it's a very nice ability, I can assure you, it's really not a huge deal.