Because he wanted to indicate that the soldiers were whispering.
Explanation:
In comic book stories the shape of the balloons represents specific characteristics of the characters' tone of voice. Balloons that represent common voice tones are drawn with a simple and normal line, however, balloons formed by a cut line mean that the character is speaking so softly that he is whispering.
These elements in the stories are important because comics do not have a narrator to indicate the tone of voice that the characters are using.
