What is the ruling on this father, to what extent should I stand with my mother against him, and what is the ruling on testifying against him regarding his death threat, knowing that the threat occurred in front of me?

Filial piety towards parents is obligatory, even if they are unjust or tyrannical. These reprehensible qualities do not negate the obligation of filial piety. Allah Almighty said: "But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them, but accompany them in [this] world with appropriate kindness" [Luqman:15]. However, filial piety does not preclude enjoining what is right and forbidding what is wrong, preventing injustice, and bearing witness with justice. Allah Almighty said: "O you who have believed, be persistently Qawwameen (maintainers of justice), witnesses for Allah, even if it be against yourselves or parents and relatives" [An-Nisa:135]. A son's truthful testimony against his parents does not negate filial piety; rather, it is part of their piety, for it saves them from falsehood.