Viet started shouting verbal abuse at Koichi with a malicious grin.
I've never bad mouthed my husband in front of my kids.
I heard that all of my friends were saying bad things about me.
Your bad mouth is saying bad things (as usual!). Everything that comes out of this bad mouth of yours is verbal abuse. One day someone's going to pop you in that bad mouth of yours.
The reading is the kun'yomi vocab readings for these two kanji, probably because this word is just two words put together into one (bad + mouth). Use your knowledge of the vocab words 悪い and 口 to learn the reading of this word.
Okay, I lied. You can also do わるぐち if you want. Sorry to say such bad things before.