Why did Khalid ibn al-Walid, may Allah be pleased with him, kill Malik ibn Nuwayra?

Malik ibn Nuwayrah withheld the sadaqah (charity/poor-due) and distributed it among his people when news of the Prophet's death reached him, and he recited poetry indicating this. It was also reported that he used to belittle the Prophet (peace be upon him) by saying, "I do not think your companion said such-and-such," which led Khalid to consider these statements an act of apostasy from Islam.