What are the consequences of civilians picking up arms (such as Molotov cocktails) to defend their cities?
By giving orders for the Russian military to fire on civilians in cities, those who issue the order and the soldiers would be culpable for war crimes.
It would appear that Ukrainian civilians have every right to defend their cities by attacking advancing soldiers using all means at their disposal.
Does civilians' defence of their cities absolve advancing soldiers of war crimes?
The question is not about the fate of the civilians, but that of the soldiers.
Let's look at a more distant example.
Under command from the Nazi or the Soviet leadership, soldiers were given orders to enter residential neighborhoods, kill some civilians, and capture others—sending them to Nazi extermination camps or the Gulag.
After the hostilities ended some soldiers and some civilians survived. The soldiers are prosecuted at Nuremberg or The Hague, and some civilians are ready to testify as eyewitnesses pinpointing the attacking soldiers. Is a defence of "but the civilians I was shooting/capturing were actively throwing Molotov cocktails on me, and hence they were legitimate targets" a viable defence against a war crimes charge?