a murderer: someone who has wrongfully taken another person's life, or who is charged with having done so
Greek keeps three closely related words for murder: the deed itself (φόνος), the act of committing it (φονεύω), and this one, the person who has done it. This is the label, and in the New Testament it is always a label somebody is pinning on somebody else: a king on the guests who killed his servants, Peter and Stephen on the crowd in Jerusalem, a group of islanders on a stranger who has just been bitten by a snake.
That is worth noticing, because the label is a moral judgment rather than a legal finding. Nobody in these verses has stood trial. In Acts 28:4 the people of Malta decide Paul must be a murderer because a viper fastened on his hand, and they turn out to be wrong. In Acts 3:14 and Acts 7:52 the word is aimed at people who did not themselves strike the blow, because it fixes responsibility for a death rather than describing whose hands did the work.
The same word can also name a whole kind of person rather than one culprit. In Peter's letter and twice in Revelation it stands in a list beside thieves, liars, and idolaters, marking out those who have no place among God's people. Greek had narrower options available if a writer wanted them: a word for the political assassin with a dagger (σικάριος), and a compound that spells out that the victim is a human being (ἀνθρωποκτόνος), which John turns on the devil and on anyone who hates a fellow believer. This word stays the plain, general one.