The Magnificent Seven: why did they warn the villain?

Spoiler alert, obv.

Why did they send the "sheriff" to warn Bogue about what they had in mind? They just could've waited until he shows up again, totally oblivious and unprepared... And then kill him off. They could have had more time to prepare the city and themselves. Did I miss something? I didn't watch the movie in my mother tongue, so it could be that there were bits I didn't understand.