In the Harry Potter series Lucius Malfoy is depicted quite early as one of the first Death-eaters. As a Death-eater, we can assume that he has probably used unforgivable curses many times (I wrote probably because, even though I don't remember that the use of such a curse is ever mentioned, in the second movie he uses Avada Kedavra towards Harry so insensitively that one can assume that it is mainstream for him).
But the use of such a curse is supposed to be worth a one-way ticket to Azkaban; my question is: why did Malfoy never get arrested for his crimes? At least, shouldn't he be fired from the Ministry of Magic if we consider his relations with Voldemort?