It would definitely turn out wrong, so I won’t be doing something so stupid.
It would definitely turn out wrong, so I wouldn't dream of doing something so silly.
「確実に変になるから、そんなアホなことしない」は英語で It would definitely turn out wrong, so I won’t be doing something so stupidになると思います。It would definitely turn out wrong, so I wouldn't dream of doing something so sillyでも大丈夫です。
もう少し詳しく説明すると If I were to cut my own hair it would definitely turn out wrong, so I wouldn't dream of doing something so silly.になります。
If I were to cut my hair by myself もしくは If I were to cut my own hair – もし自分で自分の髪の毛を切ったら…
確実に- definitely、 certainly
変になる – 直訳 become strange (自然な訳 – turn out wrong)
アホな – silly、stupid、foolish、idiotic