Let's take a completely silly example. Charming Charlie the 13th level rogue has the Diplomat feat. He's +15 to his persuasion check and because he has reliable talent, cannot get anything less than a 25 on his persuasion check. So basically he can charm just about anything he talks to for a minute.
A completely stupid scenario:
- There's a group of balors in a cave. Thirty of them.
- Charlie casts alter self (he's a trickster and swapped out a spell at 8th level) and walks into the cave.
- Charlie talks to and automatically charms each balor in turn.
- Once the creature is charmed, there is nothing that can break the charm other than moving more than 60 feet away from the charmer.
- After half an hour Charlie's group blocks the entrance. Wall of Stone should do the trick.
- The room is less than 60 square feet so Charlie can now walk around killing each demon. They can't attack him because they're charmed.
A DM can always just say "that doesn't work". The DM could say that once the rogue attacks the charm is broken. They can change the rule.
But without changing the rule, without saying "it doesn't work because I said so" there's nothing stopping it.
Yes, this is a silly example. I know several people that would argue until they are blue in the face that it would work. According to the text of the feat, it should.
A less silly example of course is Charlie talking to virtually any individual on the planet that can understand them, charming them and then killing them. If the target can't get away or summon help, they're SOL.
And 3...2...1... the response is going to be "DM empowerment" and "Rules over Rules". All I can say is that if you think this loophole big enough to herd a tarrasque through won't be abused you've never played a public AL game. Or with Jon*, one of my players.
*Not his real name