The big deal is that Rabbi Yitzhak Shapira defends killing people who "threaten" Israel or "support" war against Israel, not people who are actively trying to harm others and thus might be killed in self-defense. Shapira even defends killing children whose very existence causes problems for Israel.
According to Shapira, it is permissible to kill a non-Jew who threatens Israel even if the person is classified as a Righteous Gentile. His book says that any gentile who supports war against Israel can also be killed.
Killing the children of a leader in order to pressure him, the rabbi continues, is also permissible. In general, according to the book, it is okay to kill children if they "stand in the way - children are often doing this." "They stand in the way of rescue in their presence and they are doing this without wanting to," he writes.
"Nonetheless, killing them is allowed because their presence supports murder. There is justification in harming infants if it is clear that they will grow up to harm us. Under such circumstances the blow can be directed at them and not only by targeting adults."
I'm not sure that this can be seen as anything other than a call to terrorism -- or at least vigilantism. It would appear that anyone who ever supports any opponents of Israel could easily be labeled as supporters of war against Israel. That person could then be targeted for murder, at least if Rabbi Yitzhak Shapira had his way. The person's infant children could also be killed under the assumption that they might grow up to be like the parents.
Religious extremists certainly tend to think alike -- the defense for killing infants offered by Rabbi Yitzhak Shapira looks an awful lot like how conservative evangelical Christians justify stories in the Old Testament when God orders the killing of everyone, including children. Christians argue that God knew that the children would grow up to become enemies of the Jews, or at least awful sinners, so it was best to kill them young.