The arrest itself is alleged by the prosecutor to have been illegal
The prosecutor has to allege that the arrest was illegal, because if it were legal, then the officers have a much stronger case. Everything that I've read, by the way, suggests that the arrest was legal. Probable cause for carrying a knife that was illegal under Baltimore city statute was almost certainly established.
the very first thing in the police report regarding the reason Gray was stopped was "made eye contact".
And then ran. The cops aren't going to chase you down, snap your spine, and toss you into a paddy wagon like a sack of meat just for looking at them.
The knife Gray was carrying was not illegal.
That can only be determined in a court of law, but the knife almost certainly was illegal by Baltimore city statue, which is the statute that he was alleged to have violated.
it's probably not even illegal to run away when you see police.
It is not illegal to run from the police unless you have been given a lawful order to stop.
Do I really have to list the names of unarmed black men who have been recently killed by police just to demonstrate that running away may in fact be the smartest thing that one can do?
One thing that all of those shootings have in common is that the deceased has either run from the officer, physically attacked the officer, or both.
Running from the police is a bad idea unless you know that you will be in trouble if caught, and also you are certain that you can evade them successfully. Physically attacking an officer is always a bad idea.