Yes you did have a legal reason for holding him...an ICE detainer. Before I retired, I was responsible for prisoner releases, and if I had an ICE detainer, I always coordinated with ICE to ensure the detainer was still valid, and if it was, to arrange transportation to the facility were federal prisoners were held. There was none of this sanctuary city BS. Enacting such feel-good laws has consequences, deadly consequences, in this case. I bet the victim's family doesn't feel so good about Frisco's policy.
Rhetorical question to you, not looking for an answer! Just pointing out the disconnect in subject matter, as the incompetent parties try to deflect blame.