The burden of proof is on the state and for good reason. The police officers who asked the judge to issue this warrent should have provided more proof. They could have still arrested him on lesser charges if they felt he was a danger to himself or to other people.
Police have a tough job, and it's almost impossible to actually get into somebody's mind and know what they are thinking without serious evidence (a journal) or having them in custody for an interview. A lot of times the motive is uncovered after this sort of questioning, so not understanding the motive isn't a good reason not to issue a warrant.
Endicott McCray is suspected of committing murder and a warrant should be issued for his arrest. Premeditation and/or motive can be determined after he is placed into custody. He is armed and considered dangerous and must be placed under arrest for the protection of Austin citizens. Not issuing a warrant for his arrest could endanger the lives of many more citizens.
Judges issue warrants based on probable cause evidence. If there was sufficient evidence for the judge to believe McCray may have committed murder per the details of the law in that jurisdiction, he or she should issue the warrant. If the burden of proof was not met, there should have been no warrant until the investigation reached a point such burden could be met.