Matt was killed by members of Takada/Kira's security force after being forced off the road. He was killed because of his affiliation to Mello and was murdered in a hail of bullets. It should be noted that he was murdered by the security force and was not killed with a Death Note. It is unknown whether Kira or Kiyomi Takada ordered him to be murdered or their security force decided to kill him out of spite for kidnapping Takada.