Your confusion stems from the fact that you're not seeing the semantics at play in each language in those two sentences.

In English, we say "**do** something **to** someone" where *someone* logically is the indirect (dative, not accusative!) object, recipient of the verb **do**.
However, in Russian, the semantics change; instead of **doing** something **to** someone, they **act** (поступать) a certain way ***with*** (с) someone.  

In other words, your error here was expecting a dative based on English usage instead of parsing the Russian apart from the English.

Russian:  
**Как ты мог так поступить со мной?**  
*How you could so act with me?*

English:  
** **

Same meaning — two different forms of expression.