The first thing we have to know is that there are very few cases where "defense of a third party" is asserted in the appellate opinions. More often, it is a claim of self defense. The standard is the same however and appellate opinions on both are applicable here.
There are two inquires that must be made:
The first inquiry examines a defendant's subjective belief and requires evidence indicating that the defendant honestly and sincerely believed it would be necessary to kill [to protect a third party] because of an aggressor's imminent use of unlawful force. The second inquiry is objective and requires evidence showing that a reasonable person in the defendant's situation would have perceived the necessity of killing [to protect a third party]. State v. Walters, 284 Kan. Syl. 3, 159 P.3d 174 (2007).
Under this legal standard, Roeder has no chance of claiming "justifiable homicide."
Look at the subjective portion. For our purposes we'll grant Roeder an "honest and sincere" belief that he had to kill to protect a third party. But even under the subjective portion of test (the part that should be the easiest for a defendant to meet) Roeder has problems.
1. Dr. Tiller would have to have been an aggressor for this to even apply. Passing out the church bulletin on Sunday morning is hardly an aggressive act.
2. Dr. Tiller was not using "unlawful force." He was passing out bulletins in his church. Even if we, as the defense will no doubt argue, make this about abortion - it's not unlawful. In fact, that is the very problem Roeder and his fellow terrorists have is that it is lawful!
3. Finally, there was nothing imminent. Dr. Tiller was not in his clinic. He was not with a patient.
Even if Roeder could get past the subjective portion of the test, the objective portion would deny him his bully pulpit. A "reasonable person" (that is the average guy on the street, not the average terrorist) would never perceive that killing was necessary in this case.
The long and the short of it, we can't see any legal way that Judge Wilbert can allow this defense to be argued. Oh, Roeder's fellow terrorists will claim judicial activism, that the D.A. bought off the judge, and kinds of other idiocy. But, in truth the law will simply prevail over terrorist activity!