Do you know that MURDER is the #1 cause of death for pregnant women? Do you know that the murderer is ALWAYS the father of said pregnancy?
So, are you going for the aliens from Mars theory? The One-Armed Man? How about Johnny Cochran's glove theory?
Scott had a perfectly understandable motive for killing Laci rather than divorcing her--if he got a divorce he would still have to pay child support for their son for 18 years. His best legal strategy might have been a 14th-amendment defense (equal protection of the laws): he as the expectant father was not given the same rights to dispose of an inconvenient baby than any expectant mother would have.