There is a reserved parking space that is not mine and I'm going to take it.
What's the reason you would take the reserved parking space that belongs to someone else?
The person has been threatening me and he's very rude.
A reserved parking space belongs to someone else and if you take it, it is a version of stealing. You have no rights to anyone else's parking spot and if you park there you will end up getting towed.
What are you going to do about it huh?
There's nothing I can do about it, but I think it's wrong to take the spot that's not yours as it's rude and it will only make the problem you are having with this person worse. If this man is truly threatening you, I suggest you report him to the appropriate person depending on the severity of the threats. It's important to protect yourself.