It sounds like the man was messed up:
The suspect "had tunnel vision," said Stanislaus County sheriff's deputy Royjindar Singh, a department spokesman. "As people tried to intervene, to tackle him, he just went back to what he was doing. Anything and everything he could do to the baby, that's what he was doing."
My guess is some untreated mental condition. Still glad he's dead.
http://www.sfgate.com/cgi-bin/articl...9MI5.DTL&tsp=1