[If we're swapping canonical anecdotes: this one time, Sebastian was fighting two Grim Reapers whom he had trouble taking on one on one. Enter a third Reaper, who effortlessly defeated the other two despite fighting them both at the same time. Sebastian then decided it was a good idea to attack this Reaper with a butter knife. ...So you see his problem here. He's not very good at acting appropriately when he's the tiny ant in a situation.
And had this been a different situation, another place, if there weren't people around and Sebastian had a facade to uphold, chances are he would be happy to start this fight (or what he wrongly imagines would be a fight). As it is, this isn't a situation where he can go all out.
He lifts his left hand, removes the glove the way he tends to do - using his teeth to pull it off - and then holds up the bare hand for Castiel to see. Recognise the seal? Apparently Sebastian thinks it should tell Castiel something.]