There are many words that can be used as synonyms for the phrase "acting too hastily". Some options might include rash, impulsive, reckless, imprudent, thoughtless, precipitate, or premature. Each of these words carries slightly different connotations, and can be used in slightly different contexts. For example, "rash" might be used to describe someone who acted quickly without considering potential consequences, whereas "reckless" might describe someone whose actions were dangerous or irresponsible. Ultimately, the choice of synonym will depend on the context and tone of the sentence in which it is used, as well as the specific connotations that the writer wishes to convey.