Sandu is fully pulled from its sheath. It takes a similar position to Bichen. He's aware that this probably is going to get him in trouble for fighting, but he would rather get a punishment from Master Lan than get stabbed by Lan Wangji for being unprepared.
He glares even harder when Wei Wuxian touches Lan Wangji. Lan Wangji has always been too soft about Demonic Cultivation - has never taken it seriously enough. Has always looked down his nose at Jiang Cheng, as if his attempts to rid the world of a dangerous mind-destroying evil technique is somehow inappropriate. Vanishing off with Wei Wuxian, doing who-knows-what, getting Wei Wuxian involved - is he trying to get the Yiling Patriarch back?
"Get away from him," he growls. He's not sure which of them he's talking to.