そんなことをしたら

if you do such a thing

→ そんな
phrase Rare #37,253
if you do such a thing Used to introduce a consequence, often as a warning or threat. そんなこと means “such a thing,” and したら means “if you do.”
JA: そんなことをしたら、みんなに笑われるよ。
EN: If you do such a thing, everyone will laugh at you.

そんなことをしたら罰せられるだろう

If you do that, you'll be punished.

二度とそんなことをしたら承知しない

If you ever do that again, you’ll be sorry.

そんなことをしたら逆効果になる

Doing that would have the opposite effect.

万一そんなことをしたらあなたはひどい非難を受けるだろう

If you were to do something like that, you would face severe criticism.