When I have time I'll look into the motivation for multiple kill murderers' first kills - whether it was greater than for subsequent victims. Even in non-psychopathic murders the motive can be small - the slightest irritation.
Whether he premeditated Laura's murder, he apparently felt a thrill rather than the revulsion others might, learned that he could get away with it, and continued from there.
His post death treatment of his victims also says a lot about his personality.
The fact that he bought an incinerator to dispose of human remains is a strong indication he might have planned to carry out more murders in the future. IMO