Magnus whirled, his electromagnetic shield up in moments and every metal object in the room levitated into the air, ready to strike down whatever adversary dared disturb the sanctity of Magneto's home. All of this in mere moments before he realized that one of the infected creatures would have been unlikely to sneak up on him and even less likely to form a coherent statement. This knowledge didn't put him off his guard.
He scowled at his son and let the objects he'd move drop back into their proper places, though he didn't immediately drop his protective barrier. Probably, he ought to have been pleased to see that his son had survived and maybe then inquired as to what exactly Pietro was doing here. Mostly, however, Magnus was annoyed and so all he said was, "Give me one good reason not to kill you."