毒にあたる means "get (oneself) poisoned"
毒にあてる might be interpreted as "get someone (else) poisoned" but it is not a commonly used expression.
Normally we say 毒を盛る to get someone poisoned.
毒気にあてられる is a fixed phrase, it has nothing to do with poisoning, but "to be taken aback" or "to be appalled" to see/find/meet condescending behaviour, etc.