I see 2 possibilities.
1: You manufacture a poisoned soap bubble. But ShyGuy implied that the soap bubble was found already floating, so that's no good.
2. You use the soap bubble as a distraction. For example, you might cup the soap bubble in your hand, beckon your victim to take a close look, and then hit him. Hard. Or you might blow it in a conveinent direction so that your victim turns his back to you, and then hit him. Hard.
....Now, you might argue that in scenario 2, your arms are doing the killing more than the soap bubble. But actually, as long as we don't have autonomous killing robots, objects don't kill so much as they increase your power to kill, which they can do to varying degrees. And since a soap bubble can do it to some degree, I think it counts.
What's that? I should stop being philosophical and start being funny? Sorry.
Okay, next item: desk fan.