The level of legal understanding from some of the comments on here...
Firstly he has been arrested NOT charged. This is so that they can get him to the station and have the full time to do interviews, hold him etc. It does not mean or even IMPLY that he will be charged with murder.
Secondly: on the facts as we know them, if someone is mid murder and a passerby attempts to stop them, resulting in the murder's death, they have not committed a crimeit is a full and complete common law defence of self defence (defence of another). People on here saying 'well he died, of course he's going to get some punishment!' know jack shit. That's NOT the law. Self defence is a COMPLETE defence-means the entire case fails if it is successfully shown. The force will need to be reasonable, bearing in mind all circumstances and the fact he is acting in the heat of the moment. He didn't run over him multiple times (as far as we know) it seems an act of desperation to stop the stabbing which would be reasonable at the time. Fun fact (doesn't apply here): in householder cases it doesn't even need to be reasonable force, it just need to NOT be grossly unreasonable. So yes people do get killed lawfully under self defence.
The police will have no interest in prosecuting and the CPS will say (if these facts reported are shown to be correct) that he has used reasonable force in the heat of the moment, he would have a full defence to murder and therefore there would be no hope of a prosecution, so he will not be charged.
The whole point of the arrest at this stage is to ensure that the facts are established correctly, if they are as reported he has not committed murder, manslaughter or any other crime here.