There is no doubt Roeder committed a crime, it is up to the jury to decide which one.
Hence the reason he should be afforded a fair trial to include testifying about the abortion procedures that helped motivate him to believe his victim’s behavior was so horrible the only way he could be stopped was to kill him. Such testimony is not irrelevant. Add to that the fact that the judicial system failed when this doctor was indicted (other testimony barred by the judge), Roeder had a rational justification (in his mind) which may lead to a less severe penalty.
It’s a reasonable defense although it won’t likely be successful. Case law has ruled that simply because a defense will not likely be successful the court cannot deny someone the opportunity to attempt it.
The point is, the guy is guilty of a crime, no doubt, however, under no circumstances should his rights to present a defense be abridged simply because he is a critic of abortion and the court wishes to remain PC.