Renly was the true steel

Maegor did nothing wrong, and if Euron brings down the Hightower he'll have justified all his sins. The catspaw is time traveling bran, he's warging in the past trying to gain info, accidentally wakes up in the stables, has memories of robbing merchant and finds the bag of silver. Realizing he can't jeopardize anything before he gains the weirwood net he goes through with the murder attempt. When he says "you weren't supposed to be there" it's to summer, he's mock scolding him, nostalgic for when they were housepets