I should've known better when you started to act weird
Eminem is as sarcastic as ever, playing along with the suggestion that she is in fact the murderer. After explaining how he was going to pin the murder on her, he says “I shoulda known better when you started to act weird”, indicating that he is going to act heartbroken and surprised when they police find out about the “double homicide/suicide”.
An interpretation external to his made up scenario for the cops, would simply be her acting weird because she was hiding that she was cheating on him, which should have made him suspicious way before.
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