Because the murderer was not in proximate relationship to an immediate murder taking place.
In fact, Tiller was at church.
Let’s not overlook, however, that this could be an aggrieved family member of a Tiller victim. No one has id’d the murderer as of yet.
A man with Operation Rescue in KS was interviewed and told the reporter he didn’t know the perp “based on the name” she (the reporter) had given him, so the name is out there somewhere.
"immediate" is also a matter of perception. Monday morning could be "immediate" in the mind of the killer.
My first thought when hearing of this was to look for justified killing in Scripture. The first thing I found was Exodus 21:22-23. I read that as indicating life for life if a baby is killed.