"It's very unfortunate, extremely sad. We need to take our responsibility, make sure that our tool is being used in positive and healthy ways," Dorsey said.
In October, the dismembered bodies of 9 people were found in 27-year-old Takahiro Shiraishi's apartment near Tokyo.
Police say he admitted to the killings. They believe he preyed on people who posted about wanting to kill themselves on Twitter.
In the interview, Dorsey said Twitter is a public tool on which anyone can post messages, and he hopes that those who are considering suicide can use his company's service to connect with someone who will give them the help they need. He also said it's impossible to stamp out suicide completely, no matter what technology is used.
Earlier this month, the company changed its rules to ban users from encouraging suicide or self-harm. Company officials said they would decide on whether to delete suicide-related postings after discussing the advantages and disadvantages of doing so with people inside and outside the company.
This is the first time Dorsey has given an interview to Japanese media since assuming the post of CEO in 2016.
Suspect looked for victims on Twitter
Investigators looking into the Zama case say the suspect told them that a 21-year-old woman became his first victim because she was the first person he was able to meet through Twitter.
The suspect, 27-year-old Takahiro Shiraishi, reportedly told police he had tweeted about suicide and sent messages to various women since March.
Shiraishi was arrested for abandoning a body in late October after dismembered bodies of 9 people were found from his apartment room.
Police suspect that Shiraishi aimed to kill women indiscriminately. They believe that after getting acquainted with his victims, he switched to direct messages, which can be read only by the sender and receiver.
Police suspect that Takahiro Shiraishi killed 9 people within 2 months from late August. He is believed to have preyed on women who posted suicidal wishes on Twitter.
Shiraishi's first victim was 21-year-old Mizuki Miura from Atsugi City. Miura and a male acquaintance, 20-year-old Shogo Nishinaka from Yokosuka City, met the suspect for the first time on August 13th and had a drink with him in a park. Nishinaka was also killed in the alleged serial murder case.
Six days later, on August 19th, Shiraishi received 510,000 yen, or about US$4500, in his bank account from Miura. She deposited the money so that he could rent an apartment.
Miura also accompanied Shiraishi to a real estate agency when he signed a contract to rent the apartment in Zama.
Miura went missing on August 21st. She left a note saying she would disappear for a while but that she would be back.
Police found Miura's mobile phone in the women's restroom at Katase beach in Fujisawa City, about 20 kilometers away from the apartment Shiraishi had rented.
Shiraishi told police he killed her in late August, and added that he had rented the apartment to assault her.
15-year-old Kureha Ishihara from Gunma Prefecture went missing in late August on the first day of the second term at her high school. The school says Ishihara contacted them that morning and said she wouldn't be able to attend.
Her friends say she posted a message on Twitter, probably to herself, wondering why she did her homework so intently if she wasn't going to go to school.
26-year-old Hitomi Fujima from Saitama Prefecture went missing after speaking with a male acquaintance on September 23rd. She reportedly told him over the phone that she couldn't meet him that day because she was going to see someone she got acquainted with on Twitter. Fujima's family asked police to look for her, as she had left a note suggesting that she was going to kill herself.
17-year-old Akari Suda from Fukushima City had stopped showing up at school from September 27th. Her friends at high school say she withdrew from their group chat room on the LINE messaging app the next day. Later that day, her mobile phone stopped emitting signals after being detected in an area near the suspect's home.
23-year-old Aiko Tamura from Tokyo's Hachioji City went missing after meeting an acquaintance on the evening of October 21st. Police say security cameras at the JR's Hachioji station and the Odakyu Odawara Line's Sobudai station captured Tamura walking with Shiraishi on October 23rd.
On October 30th, police searching for Tamura raided the suspect's apartment unit, and discovered the bodies of 9 victims.
Shiraishi told investigators that he lured women who tweeted suicidal wishes to his room and killed them after a brief chat. He said none of them really wanted to die.
Japanese government to act against 'suicide websites'
Chief Cabinet Secretary Yoshihide Suga instructed ministers on November 10th to step up measures against inappropriate websites that deal with suicide.
The ministers met to discuss how to respond to the alleged serial murder case.
Suga said the suspect appears to have skillfully taken advantage of the mental conditions of his victims, who posted their death wishes on social networking sites, including Twitter. He called the acts despicable.
He said the details must be thoroughly clarified to help prevent recurrences.
Suga directed the ministers to study the spread of suicide websites and to cooperate with web operators and related authorities. He asked them to step up measures to delete or restrain problematic sites.
Suga also urged them to help young people find appropriate counseling when they express the desire to commit suicide online.
On the same day, he announced a plan to hold a meeting of bureau chiefs of relevant ministries and agencies early next week, and to compile preventive measures in a month.
He also said the government will do all it can to swiftly implement preventive measures on ministerial levels as they become available, even before the compilation.
NHK survey: Nearly 70% support restrictions on suicide websites
An NHK survey has found that nearly 70% of people approved of the recent government move to step up measures against problematic websites that deal with suicide.
It surveyed about 2,100 people aged 18 or older on the phone from November 10th to 12th. Nearly 60 percent responded.
Asked whether they think such government measures are necessary, 68 percent of the respondents said yes, 7 percent said no, and 17 percent said they were undecided.
Internal Affairs Minister to "mobilize all resources"
Internal Affairs and Communications Minister Seiko Noda spoke to reporters after a cabinet meeting last Friday. She said it had been pointed out that 50% of junior high school students and 90% of senior high school students now have smart phones, and that the younger generation appears to be part of a different realm.
She expressed her intention to mobilize all resources available to address the issue. Noda said she will call on telecommunications carriers to take appropriate measures to their fullest extent. She also said she will check current efforts for any faults and make sure inter-ministerial coordination is functioning.