Acerba sunt bella fratrum. Bitter are the wars between brothers.
Regulus sees Sirius, laughing under the large beech tree beside the Lake with his friends. Regulus liked to sit under that tree, once. Before Sirius took that away too.
It's like they don't realise there's a war raging outside the boundaries of the castle. It's like they don't realise there's a war brewing inside. For all their careless so-called pranks and hexing of his friends — hexes that would be classed as very serious and bordering on dark magic if any Slytherins were caught using them, but no, because it's Perfect Potter and his merry band of Gryffindors they get away with a mild detention or a slap on the wrist — for all their prodding and goading and hurled insults they don't know. They don't understand. This isn't a game.
The war between these brothers has grown, mutated, spun out of control. It's no longer a matter of differing opinions (and their opinions on what really mattered hadn't been so different at all) or of differing values (again, not so different; Sirius had merely had the strength to value the family he chose over the one he was born into). Now, the war is The War.
Regulus knows how this will end. He has seen Sirius and Potter lurking around the headmaster's office, whispering excitedly with that same gleam in their eyes that Evan still held whenever they were given a task or called for training. He knows about Dumbledore's resistance, knows that he, like the Dark Lord, has no qualms about sending teenagers into battle. About spilling magical blood for the cause that he believes is right.
Regulus knows that he will end up facing Sirius on the battlefield and that his brother — quicker, braver, less cautious than he — will undoubtably better him. Sirius had won any time they'd duelled in the past.
He just hopes that his mask will stay put, so that Sirius might not have to bear the shame of knowing what his little brother has become.