Claudius waits to do this. He could have stopped it as soon as Laertes stepped out of bounds and wounded Hamlet. But he doesn’t. He waits. And watches while the swords, get switched, knowing full well that the sword that is passing from Laertes to Hamlet is a murder weapon. He makes no move to save Laertes’ life. He’s probably thinking that a dead Laertes can tell no tales on him.
But I suspect his failure to prevent Laertes’ death is probably a big factor in Laertes deciding to confess.
I mean – the timing is malleable. The stage directions place this line after the mutual wounding – though certainly one could stage it so that he tries to stop it before the sword touches Laertes. That seems a little out of character for our politic villain, though.
He could also say it the way Gene Wilder’s Willy Wonka says his, “Wait. Stop.” – that is, not very loud – and without conviction.