There are several ways to kill someone, but a knife is a terrible way to die. First, using a knife makes it personal. The murderer wants to inflict pain. Stabbing someone and watching them bleed out is absolutely evil. This cop was only doing his job by assisting another man that had his backpack stolen. These thugs steal the backpack and then threaten him that if he doesn't give them 100 Euros and a gram of cocaine they will not return the bag.
The cop intervenes trying to get the backpack back and probably arrest these dudes when a scuffle breaks out, so they grab a knife and kill this young officer. They are arrested in their hotel room while preparing to leave Italy. Good thing they got them before they did.
I don't know a lot about Italian law, but after watching Amanda what's-her-name's trial, the Italians hopefully, won't screw up this case.