Some of the names on the PHD's list of "people who might want to kill Chief Johnson" are those we've learned about throughout the series. They include:
Joseph Dellamonte: Mentioned in Overkill
Gerald Curtis: A prisoner caught by Brenda in an elaborate scheme to escape from prison in No Good Deed
"Little Devil": A gang member who shot up the wrong car out of revenge in Slippin'
"Pookie": A gang member who shot up the wrong car out of revenge in Slippin'
One-Eight-Sevens: A local gang the squad dealt with in Slippin' and Four to Eight
Alonzo and Delia Lopez: Brenda arrested Alonzo for the murder of his son-in-law, who was abusing Alonzo's daughter, in Standards and Practices. His wife, Delia, was complicit in the murder, but was not charged.
Rawley: The Rawley family played a prominent role in The Other Woman, as Sammy Rawley was charged with murdering a woman he thought was his sister. Sammy's sister, Gennifer, was a severe drug addict that Sammy believed was destroying the rest of his family, therefore, he wanted to stop her from causing more harm.
Carlos and Miguel Torres: Brenda, along with the entire PHD, especially Sanchez, was able to arrest Carlos for the murder of two gang members, one of whom was Carlos' nephew. His son Miguel was arrested for, among other things, gang activity, which was what Carlos was desperately trying to prevent in Four to Eight.