A bloodied Sebastian easily numbered among the scariest things Bard had ever seen, and this from a man who'd gone to war (and whose job still included a fair bit of violence.) It wasn't that Sebastian himself was frightening, he wasn't any kind of berserker, dangerous to friend as well as foe. The problem was that the butler was so damn terrifying.
That didn't make any sense did it? Not scary because he's scary, but because he's scary. Just, just, stick with this. It'll make sense eventually, it's just that the whole thing is a bit confusing, even in his own head, and Bard will be the first to admit that he hasn't really planned this thing out. Not his style.
Anyways, no - no, Sebastian wasn't frightening because he might hurt someone from the household (minus, perhaps, a servant who'd just mucked up their chores moments before guests arrived, the man had this god awful look, one that promised agonies unmentionable for those kind of situations.) Sebastian was frightening because he was a force of nature. Bard had seen very little of the man's actual fighting capabilities, had no idea what he specialized in, if he even did. That didn't mean he hadn't pieced enough together to hope that the butler never really blew a gasket.
The man was preternaturally fast, snagging mice right out of the air and easily catching the stacks of plates Mey-Rin was prone to dropping. Strong enough to throw a hell hound around almost as easily as Finny did, and sturdy enough to take a plummeting statue in the back. Hell, he was good enough with a foil that the duel with Agni, that Indian butler, had ended in a draw. Bard didn't know much about fencing, for upper-class nob's that was, but he was pretty sure those two had put on a spectacular show.
And he did it all with that placid calm, the mysterious and absolutely unflappable demeanor that he always held about him, like some sort of cloak. (Bard was pretty sure it was the half-lidded eyes. He'd thought, at first, that it was the smile, but half an hour in front of a mirror contorting his mouth into different shapes and he was sure. Definitely the eyes.)
So yeah, if you were a bad guy, on the wrong side of the Queen's guard dog and unlucky enough to have his guard dog sicced on you, then Sebastian was utterly terrifying. A hound who was right out of hell, surely as the family dog. (Damn, sometimes he stopped and looked around a bit and thought 'my life is weird.') Bard knew that anyone Sebastian was sent after ended up gone. Gone, gone. He didn't doubt if for a minute, not even a second, because for all the mystique the butler so carefully cultivated with those half-lidded eyes, Bard knew him. Not as well as he knew Finny or Mey-rin, but knew him just the same, because they lived together and worked together and for all the effort the butler put in, there was a limit to how enigmatic anyone could be under those circumstances.
Bard knew that Sebastian was better at cooking deserts then entrée's (the first he did from scratch without consulting anything, the latter he occasionally browsed the cookbooks for,) but didn't really care for sweets. He knew that Sebastian's favorite cats were little black ones, for all that Ciel said he'd been bit happily hugging a tiger. The butler actually preferred dark colors, even on the all but unimaginable occasions he was out of uniform, and was actually unsettled being in messy places, especially when he couldn't immediately set to cleaning, and that, beneath the gloves, he kept his nails an unsettling, matte black for some reason. Bard even knew, and he was proud of this one because Sebastian clammed up faster than a… than something really fast whenever his past was mentioned, that he had once had a friend who favored a red cravat.*
You're thinking that he's wandered pretty far off point, but he hasn't. The point is that as scary as Sebastian might be, Bard knows him and isn't afraid of him. So when Sebastian came up the front steps and sealed the main doors behind him, actually stumbling and he made his unsteady way into the entry hall – when Bard caught sight of the blood soaking his chest and arms and even the back of his head and noticed the tension, more aggression then pain, singing along the muscles of his hands and jaw. When Bard realized that the scary-ass butler he'd worked with for years was hurt and angry his first, gut reaction was fucking terror. Because anything that could do that to Sebastian was going to be a problem.
Bards second reaction, incidentally, was a grim kind of rage-pleasure because when Sebastian stiffened painfully while climbing the stairs, Bard got the idea that this was going to be the kind of problem he'd take a lot of joy in putting holes in.
*In retrospect, red in black butler usually means Grell, but this was actually a reference to Hellsing. I don't know why, but in my head cannon Alucard and Sebastian know each other. They're probably not actually friends, that's another misinterpretation on Bard's part, but I imagine they love screwing with each other. Mind games through the ages.