Suicide is a crime that involves causing one's own death. It is different from murder, which involves killing another person. Although it is considered a victimless crime, many have argued that such a term is far from the truth, because there are many instances when a family member or a loved one of a person who commits suicide has fallen into depression and great grief over the pain of losing someone to suicide. One could also say that the person who commits suicide is the victim of their own crime.
Vigilante was a superhero who committed suicide just so that he could end his life so that he could stop feeling pain about the terrible things he had done over the years. Even though he had killed evil men, he still felt much grief, feeling like he was no different than them for resorting to such violence. He thought of himself as a murderer and decided he didn't deserve to live anymore, so he shot himself in the head with a gun.
Task Force X employs a group of super powered men and women, who go out and do dangerous missions for the government. These are often considered "suicide missions," therefore the group is referred to as the "Suicide Squad."