The Tyrells and Olenna would not get away with murdering her precious boy either. They thought themselves safe in Highgarden and might be right now. The problem is that Baelish was now officially missing and had abandoned his post, and she could not truly pin any blame on them without any proof whatsoever. And the letter from the messenger did not have anything that could be directly linked back to Highgarden or the Tyrells. So once again, she had to take things into her own hands.