歩きスマホ

スタッブ - Stop

歩き (ある) - Walk, Walking

スマホ - Smart phone

次 (つぎ) - Next

は - Particle for the Subject, Topic of the sentence

あなた - You

が - Particle for the doer of the action

加害者 (かがいしゃ) - Assault; Damaging (someone)

者 (しゃ) - Person

に - Particle

なる - To Become

かも - May, Might, Perhaps, Maybe, Probably

Translation: Stop walking with your smart phone. Next time you might become a perpetrator.

A better translation would be, 'Stop walking while looking at your smart phone. You might just kill someone.' The poster uses 歩きスマホ which doesn't include 見に to look at, but implies it. Because what else do you do with a smart phone? You stare at it and you touch it. This has become such a big problem all over the world, even in the train stations of Japan. 

'DON!' is the onomatopoeia for hitting or running into something. It's used by comedians and for comedy purposes mostly, but a little more sinister in this context.

以上です。