Long story short, a neighbour dispute about shared access to properties has been ongoing with my parents. My parents sre 100% backed up by law, police and lawyer confirmed.
Right. So that is in hand albeit by means of civil law rather than criminal.
The bloke now realises he can't win, has threatened my parents with death (police incident 1) and today keyed their car (police incident 2), it was caught on CCTV and has at least one independent witness. The witness has now been threatened. The police are going round tomorrow to confront him.
Threats to kill is an offence and he can be arrested for it although what you are arrested for and dealt with for can often differ.
For example, a complaint may be made of threats to kill but such an offence will always have to go through the CPS who may say there is enough evidence for a charge or that there is enough for a lesser public order charge such as Section 4 of the Public Order Act for causing fear or provocation of violence.
Alternatively, there may be insufficient evidence to proceed at all if there is insufficient evidence.
Keying the car is criminal damage and if it is caught on CCTV then all the better.
Threatening the independent witness, indeed any witness, is a very serious offence and if the evidence is there for a charge, it is usually accompanied by an application for a remand in custody.
What will the police do, arrest him? What crime has he commited, vandalism? and what can he expect to get, fine?
Going by what you say he seems to have committed criminal damage, threats to kill / public order offences / harassment ( if more than two occassions ) and witness intimidation.
What police charge with, assuming the evidence is there, and what the punishment is at court is a lottery and something that the cops have no control over.
Below is not aimed at the OP.
Also, and it is part of the course sometimes, is that there is often insufficient evidence to make a case no matter how hard the cops try to build one and people can walk as a result.
Some people should remember this before they **** the cops off.
At those who think they have had bad service. Complain to the relevant authority. It's that simple.