Wow that was really dark (spoilers)
When Mackey slit Woody's throat and killed him, man i didn't see that coming. What makes it even sadder is we saw him earlier in the film hugging his mom and trying to make her feel better cause i'm guessing her husband had just died recently.
So his mom is already a mess mourning that loss, now she has to find out her son has been killed, its just all kinds of fucked up and i can't even imagine the trauma and shit that brought on finding out her son has been killed.
But kudos to the writers for having the balls to do such a dark unexpected move like that.