Indeed, he was involved in a terrorist attack against a synagogue, as the getaway driver.
Let's not defeat by analogy here. He was not a guerilla fighter that assassinated people in order to stop an oppressive regime. He was not a run of the mill criminal.
He was part of an operation to kill Jews, because they were Jews, in an attempt to bring upon genocide and misery. This is not a philosophical question; it is a question of experience. In different places of the world, your hear a very good argument that he's the only kind of criminal that deserves the death sentence, possibly the most heinous type of person imaginable.
Interpret my comment more in general as a plea for the term we should hold people accountable for what is recorded in our digital memory than a plea for forgiveness of terrorist.