Is it permissible to seize state property (such as computers and cars) and sell it for paltry prices, especially if the sale is for the purpose of purchasing weapons, and is it permissible to benefit personally from it?

What the brother did by assaulting public funds and plundering them is impermissible. Public funds have a sanctity akin to that of private funds, but even greater, because it is an assault on the rights of the entire society. There has been a severe warning against anyone who wrongfully delves into Allah’s wealth, that they shall have the Fire on the Day of Resurrection. He is obligated to return what he took wrongfully to the entity from which he took it. If that is not possible, he should donate its value to the poor and needy, or spend it on general Muslim welfare.