If someone says that they are “killing time”, it might sound strange at first, but it just means that they are wasting time. Often it means that they are doing unimportant activities while waiting to do something.

“What are you doing right now?”

“Oh, I’m killing time, playing video games for a few hours until the party starts.”

日本語でも「暇つぶし」と同じ意味を表現する言葉がありますね!英語でも欠かせない作業や、大切な作業ではなく、あくまでも暇を弄んでいる時に行っている行動の事を聞かれた時には、

「用事までの暇つぶし中なの!」という軽い表現で、I’m just killing time. という表現をよく使います。