Lucius was ready to kill Dumbledore, because the old fool as chief warlock refused the military to interfere during the war and finish the war with the help of the professionals. In case of a war, the magical unit of the military was allowed to interfere, if the chief warlock, the minister and the head of the DMLE were approving their invitation. He remembered that Minister Bagnold and the former head of the DMLE, Bartemius Crouch Sr., had approved the military intervention, but the chief warlock, Albus Dumbledore, disapproved and with the disapproval of the chief warlock, the military wasn’t allowed to enter, which had costed thousands of people’s lives. If the public knew about this, then Albus would have faced an angry public and the worst backlash that would have destroyed his life completely, but no one bothered to point it out since James Potter and Sirius Black were back then his strongest supporters and were also aurors, who had arrested anyone, who had been against Dumbledore, but now he could do this since James had lost everything and was hanging on his job on a thin thread and Sirius resigned since his disownment.