Usually a "but" should be preceded by a comma:
Spooky had her kittens, but she wasn't finished her hunting career just yet.
Some people like to put a comma after "but" when "but" starts a sentence.
But, the kittens were lonely while she hunted.
I hear that's the correct way to do it.
However, not having a comma after "but" can make a sentence punchier.
But Spooky prowled the night anyway.
When I'm editing and a "but" begins a sentence, I let the author do it however they see fit. My job is to make it consistent throughout the text.
Some editing rules can be bent for stylistic purposes. If it works, it works.