Nope, not a valid defence at all.
Just for the sake of argument, let’s look at an impossible scenario.
Let’s say that you were sent back in time to 1910 where you met a young struggling artist in Linz named Adolf Hitler. You knew what a monster he would become and how many millions would suffer and die at his hand.
You also knew that killing him would be cold-blooded murder. Would you do it knowing that this would not be a valid defence?
Roeder murdered him. He should suffer the consequences of that murder. What he prevented and what he caused is not at issue here. If he truly believes that he saved thousands of lives with his actions, then death is not that much of a penalty for him. No punishment would be if he held that conviction in his heart.
The “justification” doesn’t remove the perp’s knowledge of the wrongness of his actions. If he didn’t have the capacity or knowledge of the wrongness of his killing Tiller, there might be mitigations.
Committing an illegal act to end a great evil does not make the illegal act LEGAL - just perhaps makes the illegal act RIGHT. The illegality is the source of required punishment.