Philippine President Rodrigo Duterte threatened on Monday to kill human rights activists who criticize his murderous and illegal war on drug dealers and users.

“The human rights [activists] said I ordered the killings,” he said in a speech in Malacañang. “I told them ‘OK. Let’s stop. We’ll let them [drug users] multiply so that when it’s harvest time, more people will die.”

Then he added: “I will include you [in the killing] because you are the reason why their numbers swell.”

Some 5,000 people have been killed since Duterte’s drug war started in July. Two thousand were killed in police encounters, and another 3,000 in vigilante-style slayings. Under his orders, anyone who is accused or suspected—whether correctly or not—of using, holding or selling drugs is to be killed on sight. No follow-up investigation is made into the killings to ascertain whether the accusations were justified.

No one wants drug trafficking or the crime and misery that accompany these egregious issues to continue in the Philippines. And the nation certainly needs a leader who will take a tough stance on its drug and crime problems. But Duterte’s willingness to circumvent law, and to deny due process in order to tackle such problems creates a slippery slope. Once a leader decides to bypass law for one issue, what is to stop him from doing the same for another issue? If he would kill human rights activists who criticize his extrajudicial murder of drug dealers and users, what comes next? Killing those who obstruct the murder of human rights activists?

And after them, who might the next target group be? Alcoholics? Bad drivers? Maybe Sabbath keepers?

These questions call to mind a poem by Martin Niemöller, a German anti-Nazi Lutheran leader. Niemöller’s criticism of Adolf Hitler eventually landed him in concentration camps, where he spent the last seven years of Nazi rule. His poem is critical of the German intellectuals who refused to stand up against Hitler’s purging of group after group from German society—including drug addicts, beggars, Jehovah’s Witnesses, pacifists, mentally ill, Gypsies and, of course, Jews:

First they came for the Socialists, and I did not speak out—
Because I was not a Socialist.Then they came for the Trade Unionists, and I did not speak out—
Because I was not a Trade Unionist.Then they came for the Jews, and I did not speak out—
Because I was not a Jew.Then they came for me—and there was no one left to speak for me.

Maybe Duterte is joking with his threat to kill human rights activists, and maybe after the narcos are killed, his purge will be over. But Duterte has recently compared himself to Hitler. In such a mindset, some believe his purge could expand to include other societal groups. The question in such a scenario would become the same as that in Niemöller’s day: Who will speak out?