Scott Roeder's justification for killing Dr. George Tiller is the same as the one that George W. Bush used to justify the invasion of Iraq in 2003.

In law, in order for a defense of necessity (voluntary manslaughter) to apply, there must be an imminent lethal threat to oneself or others that a person honestly believes can only be thwarted by using deadly force. But the judge ruled that this defense was not open to Roeder because Dr. Tiller was not threatening him or anyone else when Roeder cold-bloodedly murdered him.

By similarly strained reasoning George W. Bush attacked Iraq on the theory that Saddam Hussien had done bad things in the past and that he might sometime in the future threaten us or his neighbors with WMDs. There was no imminent threat that could only be thwarted by invasion. As it turned out there were no WMD's either. 

Both Roeder and Bush conveniently forgot that the threat must be imminent and that use of force must be the only way to thwart it. The main difference is that for killing one person using this absurd moral theory Scott Roeder will likely go to prison, while for killing thousands George W. Bush goes free.