People have trouble disentangling the problematic behavior of the police and the consequences of prosecutorial discretion.
The OP's argument is likely that the police didn't arrest an attacker because they knew that the DA wouldn't do anything. There is the plausible alternate explanation that the police were engaged in a bad-faith work stoppage or slow down. The third explanation--nonunique to DA--is that the police were overworked or incompetent.