I agree - if i err on the side of zero premeditation, I think something arrived in the post on Saturday, Suzanne opened it and was shocked at what she read (something financial related to one of the properties). She confronted Barry, an argument ensued and then...well we know what happened next. He then took the letter(s) as part of his clean up, as he didn't want anybody to find them. The fact that the alleged sloppy alibi (colleague) has now been made public, this further points to a rage/spur of the moment killing as it just wasn't thought through, at all! Maybe i'm just giving Barry more credit than he deserves though...
I still haven't ruled out premeditation though - based on the sheer convenience of the girls being away on a trip and the ALLEGED issue with their home security cameras.
Edited to add JMO