The 38-year-old said he was "honestly sorry" and hoped families would find peace if he was convicted, news agency DPA reported.
Identified only as Niels H under German court reporting rules, the accused told the court he acted on impulse.
"Usually the decision to do it was relatively spontaneous," he said.
Niels H has been on trial in Oldenburg in the north of Germany since September and is accused of murder and attempted murder.
The deaths occurred at the Delmenhorst clinic, where the man worked in the intensive care unit between 2003 and 2005.
He is alleged to have killed three patients and attempted to kill two others during that time, using a heart medication that lowers blood pressure.
A psychiatric expert last month said Niels H had admitted to the crimes and also claimed to have over-medicated another 90 patients, 30 of whom died.
His plan was to trigger medical emergencies so he could resuscitate his victims but he also acted out of boredom, the court heard.
The defendant said: "There was a tension there, and an expectation of what would happen next."
He claimed that each time someone died, he decided he would never do it again. But his determination would then slowly fade, he added.
Prosecutors are seeking a life sentence for Niels H, who at a first trial was convicted of attempted murder in 2008 and imprisoned for seven and a half years.
A German former nurse, who confessed to killing more than 30 patients by administering lethal drug doses, has apologised to relatives of his victims.