Only the relationship between Amanda and Meredith was perfectly good, without animosities. Amanda was in the middle of romance with Raffaele, spending every evening with him. They finished watching "Amelie" and suddenly decided to "get at Meredith". Yeah OK.
Well in SMK's theory, he thinks that they had some argument in when they saw each other that afternoon. So that would not be "get at Meredith" just for the fun of it, with no reason. I realize it might sound haste, but well aren't most, make that all, murders, burglaries, rapes, done with unsound reasons?