The situation is actually far more complicated than that.
Mr. Grant was not shot in the head, but in the lower back - and he actually would have survived the initial bullet wound, according to testimony, if the round had not ricocheted off the poured concrete beneath Grant into his lung.
Also, pretty much every witness agrees that Ofc. Mehserle was not holding him down but was attempting to get Mr. Grant's hand out of his waistband and into cuffing position.
It was a fluid situation which was out of Ofc. Mehserle's control as he tried to restrain a suspect in the midst of a crowd that was turning on him.
I don't think he intended to murder Grant, but I do think he was extremely angry and frightened and did intend to shoot him - in order to neutralize Mr. Grant's attempts at resisting and to make the gathering crowd think twice about rushing the officers.
A crowd of angry drunks on New Year's Eve is a crowd that will not be deterred by a single tasing.