Which of these words best describes an event involving indiscriminate killing?

The term that best describes an event involving indiscriminate killing is "massacre." A massacre denotes a brutal or savage killing of a large number of people, often without provocation and usually in a systematic manner. This word embodies the severity and lack of discrimination in such acts, highlighting the tragedy and violence inherent in the event.

In contrast, while "conflict" refers to a disagreement or struggle, it does not inherently imply indiscriminate killing; it could involve various forms of disagreement including debates or negotiations. The term "resolution" implies a solution or conclusion to a dispute, which again does not relate to acts of indiscriminate killing. "Celebration," meanwhile, denotes a joyous event or activity, which is entirely opposite to the connotation of violence and indiscriminate death. Therefore, "massacre" is distinctly the most accurate term in this context, capturing the essence of indiscriminate killing with its grave implications.