There were a lot of things that Snape blamed himself for - from eavesdropping where he shouldn't have, to telling Voldemort of that prophecy so long ago. But he'd done his best to keep Lily safe despite all that, had warned them and gotten them into hiding. That Potter had chosen a secret keeper who'd betrayed their trust --
That was one thing he couldn't blame himself for. Only Sirius. Who was, of course, refusing to take the blame for something he'd done. It wouldn't be the first time.
He snarled, flicked his wand in a slash and it tore up the wall behind them, just barely a warning. Severus had always excelled in the art of wordless spells -- wandless too, although that was no problem here. "No," he said, "You don't -- she trusted you." And it hadn't been deserved. He cast the spell again with a sharp, angry movement; no kind of warning. Sectumsempra was a messy choice for a spell in such close quarters and Severus was usually better known for thinking things through but sometimes even he could fly off the handle.